Site deaths are one of the big reasons why you should own your own identity and content on the web.
This is a chronology of content hosting sites that have died, terminating millions (billions?) of permalinks, and breaking the web accordingly. This specifically for content hosting sites which permitted end-user posting. Not random sites nor "app" sites (e.g. if Gmail went away we wouldn't list it here).
This is a partial list, please add more, e.g. from other sources like:
Expected/announced site deaths. Move to past as site deaths occur. Soonest first:
- 2013-08-05 Yahoo! Astrid (task tracking service)
- Astrid’s team has made a data export tool available and pointed to former competitors Wrike, Wunderlist, Sandglaz, and Any.do as potential replacements. "First off, we want to sincerely thank you for being an Astrid user over the years. Your support every step of the way has meant the world to us. As of August 5, 2013, we will be closing the Astrid service." First off, we want to sincerely thank you for being an Astrid user over the years. Your support every step of the way has meant the world to us. As of August 5, 2013, we will be closing the Astrid service.
- 2013-09-28 Yahoo! Local API (citation needed)
- As part of this shutdown, all Yahoo! Local API documentation will also be removed from the Yahoo! Developer Network portal.
- 2013-09-28 Yahoo! Term Extraction API (citation needed)
- We are eliminating direct access to the Yahoo! Term Extraction API and as of September 28, will require developers to go through YQL. We encourage all existing users of the Term Extraction Legacy API to migrate to YQL requests by September 28. You can use the YQL forums for any questions you might have about migrating to YQL. If you are already using the Term Extraction API via YQL, you don’t need to take any action.
- 2013-11-05 Everpix photo content hosting silo announces shutdown, immediately goes "read-only", and says they will "email instructions to all Everpix users on how to download their photos".
- 2013-12-15 Everpix will go offline. 
- 2014-01-02 Ptch will shut down. Ptch Joins Yahoo. Instructions provided for how to save user content.
- 2014-02-01 myOpenID shutting down, site already doesn’t load as of 2013-09-04. No public notification yet, email quoted here and here in full
- 2017-04-30 Google and Facebook "might completely disappear"
This is an incomplete list. Please help by adding examples from these sources (in particular when user authored content was lost):
See also (and add one-line summaries to) the history page.
In reverse chronological order:
Please find URLs that substantiate these shutdown, cite accordingly, and move to the respective year section below:
- 2013-06-28 Yahoo! Axis (citation needed)
- If you installed the browser plug-in, it will no longer work. If you downloaded the app, it will continue to work, but won’t be actively maintained. We encourage you to use the Yahoo! Search app for iOS and Android.
- 2013-06-28 Yahoo! Browser Plus (citation needed)
- To see all of our current developer offerings, please go here.
- 2013-06-28 Citizen Sports (citation needed)
- You can still get all the latest sports news on Yahoo! Sports, play fantasy sports like Fantasy Football, and stay up-to-date when you’re on the go with our new Yahoo! Sports app for iOS and Android.
- 2013-06-30 Yahoo! WebPlayer (citation needed)
- 2013-07-01 FoxyTunes (citation needed)
- To see the latest in the music world, please visit Yahoo! Music.
- 2013-07-01 Yahoo! RSS Alerts
- To continue to get the latest content that you care about, you can subscribe to Keyword News alerts at our Yahoo! Alerts and receive them via email.
- 2013-07-08 Yahoo! Neighbors Beta
- You can visit Yahoo! Local Search to find out what’s going on in your neighborhood.
- 2013-07-08 AltaVista
- Please visit Yahoo! Search for all of your searching needs.
- 2013-07-25 Yahoo! Stars India
- To stay up on all your favorite celebrity news, check out Yahoo! India OMG!.
- 2013-07-31 Yahoo! Downloads Beta
- Yahoo! Downloads will no longer support 3rd party downloads. It will continue to offer downloads of Yahoo! products like Yahoo! Toolbar or Yahoo! Messenger.
- 2013-02-07 EveryBlock.com ultralocal news/information site shutdown with "zero warning". Farewell, neighbors
- ???? friends lists lost. 
- ???? discussion threads lost. (ibid)
- 2013-01-30 Yahoo! Voice Phone In and Phone Out feature shut down
- 2013-01-22 Mixel  - social network shut down. (Mixel has been acquired by Etsy)
- App removed from Apple's App store, network shutting down early September (2012-09).
- current Mixels can be shared, bulk export functionality on its way
- ???? user profiles lost
- 2012-12-24 Yahoo! Messenger Public Chat Rooms, Yahoo! Messenger Pingbox, Yahoo! Messenger interoperability with Microsoft Windows Live Messenger, shut down
- 2012-12-03 Twistory 
- Not accepting new users as of 2012-11-07
- Paying users were no longer be billed, and had "several months" to download a .csv of their data
- 2012-10-31 Batch.com photo sharing site shut down. article
- 2012-10-12 DailyBooth photo sharing site shut down. article
- 2012-09-02 picplz - last day to login and download photo archives.
- 2012-07-03 site already shut down
- 2013-08-02 domain not found
- 2012-07-20 KLIK - last day to login and download photo archives
- Going down with the Face.com API due to acquisition by Facebook.
- 2013-08-01 domain not found
- 2012-07-05? Plazes downloads.
- Export was available (for who knows how long) - "you can download and keep your Plazes history" (checked as of 2012-07-05)
- 2012-06-30 site officially shut down. See below.
- 2012-07-03 picplz has shut down.
- 2012-09-03 "Photo archives will be available until September 3, 2102" (ibid)
- picplz shut down planned 
- 2012-06-30 Plazes. 
- Export available (for who knows how long)
- 2012-06-30 Apple MobileMe Gallery, iDisk, iWeb downloads.
- 2012-06-30 Apple MobileMe Gallery, iDisk, iWeb,,email
- as of 2012-07-05 on http://www.me.com: "For a limited time, you can still move your account to iCloud, download your photos from Gallery, and download your files from iDisk."
- who knows how long you can still get your data.
- "Closed." screenshot(on Flickr).
- 2011-06-06 Apple Support Communities: What happens to iWeb when there's no Mobile Me?
- Note first comment: "FYI, Apple is deleting some topics pertaining to people voicing their opinions on iWeb and iDisk. Just be careful."
- 2012-05-30 Tabblo shutdown by HP. Was at http://tabblo.com
- ??? number of photos lost (Archive Team has "almost everything" and is posting them to Archive.org)
- ??? number of "tabblos" (collection of photos) lost
- ??? number of links to photos / tabblos broke
- 2012-01-31 All original Tabblo employees gone (from HP) as well as those they directly hired.
- 2007-03-23 HP acquired Tabblo.
- "five million pictures" (at time of acquisition)
- 2006-06 Tabblo launched. (ibid)
- 2012-05-01 Google Knol discontinued - was at knol.google.com
- 2012-10-01 Download your Knol - til 2012-10-01.
- ??? number of Google Knol pages lost
- ??? number of links to Google Knol pages broke
- 2008-07-23 Google Knol launched to the public.
- 2007-12 Google Knol private beta launch.(ibid)
- 2012-04-30 Google Wave shut down.,
- 2012-01-31 Google Wave went read-only.(ibid)
- ??? number of Google Waves lost
- ??? number of migrated Etherpads lost
- ??? number of links to Google Waves broke
- 2010-08-04 Shutdown implied: "We don’t plan to continue developing Wave as a standalone product, but we will maintain the site at least through the end of the year..."
- 2012-03-11 Gowalla shut down
- ??? number of user profiles lost
- ??? number of user check-in permalinks lost
- 2012-03-01 Google Friend Connect retired for all non-Blogger sites - was at www.google.com/friendconnect
- Export your community data - unknown how long it will be supported.
- limitation: "Export Community is limited to your site’s first 2000 members" (ibid)
- encouraged to transition to Google+: "create a Google+ page and place a Google+ badge on your site".
- 2008-05-12 Google Friend Connect launched.
- 2011-07-11 tr.im URL shortener failed.
- ??? number of tr.im short links lost
- ??? number of links to tr.im URLs broken (across the web, Twitter, etc.)
- 2009-12-31 redirecting promised thru this date (and worked)
- 2009-08-10 went read only. short link creation disabled.
- 2010-12-10 Brightkite shut down - was at brightkite.com
- ??? number of user profiles lost
- ??? number of user check-in permalinks lost
- 2010-09-30 Vox shut down - was at vox.com.
- 2010-09-15 Vox went read-only (ibid)
- ??? number of Vox blogs / posts lost.
- ??? number of migrated Pownce profiles lost.
- ??? links to Vox posts broke.
- 2010-06 icanhaz.com shut down URI shortener
- Had previously stopped accepting new URIs but continued to serve old ones
- 55437 short URIs lost
- 2010-05-15 Etherpad.com offline taken offline by Google. (including their own blog post of their transition: http://etherpad.com/ep/blog/posts/transition-update). Etherpad went read-only a month beforehand (2010-04-14).
- ??? number of Etherpad pages lost.
- ??? links to Etherpad pages broke.
- Migration advised: to Google Wave. "Current EtherPad users will also be getting invites to try Google Wave in the near future...", "EtherPad users got invites to use Wave..."
- 2010-05-14 Twine taken offline at twine.com by Evri and redirected to evri.com.
- 2010-03-11 Evry acquired Twine.
- ??? number of Twine pages lost.
- ??? links to Twine pages broke.
- 2008-10-21 Twine launched publicly
- 2009-12-31 Radio Userland service closing. 
- ?? number of blog posts lost
- ?? hosted comments and trackbacks lost
- 2009-12-29 Haloscan commenting service shut down.
- ??? number of comments lost
- ??? number of links to comments broken
- ??? number of blogs with embedded Haloscan commenting broken
- 2009-08-24 Microsoft Popfly went offline
- Warning given on 2009-07-16
- ??? number of user-created sites gone
- 2009-04 Dodgeball.com shut down by Google
- ??? number of Dodgeball profiles gone.
- ??? number of Dodgeball venues gone (user created)
- ??? number of venue reviews gone (user created)
- 2009-02-28(?) went read-only at "end of February"
- 2009-01-14 Shut down announced by Google
- Subsequent site: Google Latitude.
- 2009-02 Pownce.com went offline. Was previously scheduled to go offline 2008-12-15, but voluntary code patches/contributions from Tantek helped them to transition to public read-only access (to increase chance of archival of content) on that date for about 2 months.
- 2008-12-15 went read-only.
- ??? number of Pownce profiles gone.
- ??? links to Pownce posts broken.
- Migration Advised: import to Vox: "Six Apart is encouraging Pownce members to join its blog platform Vox".
- 2009-01-30 Ma.gnolia.com shutdown.. Ma.gnolia.com suffered catastrophic database corruption and lost all user data.
- 2008-03 Consumating.com shut down by CNET.
- ??? number of Consumating profiles/posts lost.
- ??? links to Consumating posts broken.
- 2008-10-31 AOL Hometown shutdown  by AOL.
- At least 14 million sites lost.  2002 stats estimate 14 million sites on Hometown that year.
- "Well it looks like AOL did it again. Another sneaky move. The Hometown pages all seem to have disappeared. [...] I only discovered this morning that AOL pulled the plug on Oct 31st, 2008." - taimantis.com, November 11th, 2008
- "Lots of information on AOL Hometown very important to people who put hard work into these efforts is being lost, possibly without notice." - David Dillard, "Meet the Googles" Visits the Grave Site of AOL Hometown to Ponder Its Past All to Short Life
- "I was surprised to find that AOL just shut down their homepage service on Oct. 31st. There were many great websites created through that service that simply no longer exist." - Dr. Jitters, November 12, 2008
- "I knew this was coming, I just didn't know the day. I tried, with the help of some great people, to get AOL to donate ficlets to a non-profit, with no luck. I asked them just to give it to me outright since I invented it and built it with the help of some spectacular developers and designers. All of this has gone nowhere. [...] I'm disappointed that AOL's turned its back on the community, although I guess I shouldn't be surprised." - Kevin Lawver, Ficlets Est Mort
- "A terrible thing happened recently. You might have missed it. AOL Hometown, which itself was actually a combination of a bunch of previously acquired websites, shut down. [...] It's all fine and good, those readers who sneer and say 'you get what you pay for' and 'ha ha, losers'. But the fact is, these people were brought online and given a place for themselves. Like a turkey drawn with a child's hand or a collection of snow globes collected from a life well-lived, these sites were hand-made, done by real people, with no agenda or business plan or knowledge, exactly, of how everything under the webservers worked. They were paying for their accounts, make no mistake – this was often provided to them as a tool combined with their AOL accounts. Some were absorbed from other companies as AOL purchased them. Some of these websites had existed for a decade. [...] We're talking about terabytes, terabytes of data, of hundreds of thousands of man-hours of work, crafted by people, an anthropological bonanza and a critical part of online history, wiped out because someone had to show that they were cutting costs this quarter." - Jason Scott, Eviction, or the Coming Datapocalypse 
- 2001 Desktop.com shutdown suddenly, without users being able to download their documents.
- ??? user profiles lost
- ??? user documents lost
- 2001 SixDegrees.com (wikipedia), one of the first social networking services
- 2000-07-02 PlanetAll shutdown by Amazon.
- 1.5 million user profiles lost including address books, calendars, buddy lists. (ibid)
- 2000-06-06 Amazon told users "PlanetAll.com will be going away"(ibid)
- 1998-08-04 Amazon bought PlanetAll 
- 1996-11 PlanetAll launched
- Wannadoo/orange personal sites
- Why - other problems with using and depending on 3rd party sites
- Google Graveyard (on pinterest.com)