CiteULike

 CiteULike  was a UK-based silo social bookmarking service with a bent toward research and the education space.

Functionality
While the primary functionality of the service was to allow users to easily bookmark and share research articles, it also supported some of the following related functionalities:

We've removed the "long tail", so you'll only see neighbours who share at least the median number of articles of all their neighbours. You can also see all neighbours."
 * Library listing of bookmarks including the ability to sort by read/unread, reading priority, post date, number of times posted, rating, among many others expected like author, journal, date published, etc.
 * search
 * tags
 * export
 * user profiles
 * list of user's own publications
 * blog
 * groups (often use by labs, research groups, etc.)
 * connections (aka friends/followers)
 * recommendations (favorite bookmarks)
 * neighbors - "These are the users who have bookmarked the same articles as you.
 * watchlist
 * research areas: A searchable list of users who have defined their fields of research, included primary/secondary areas for discovering new content and potentially creating new friends/followers

Export
At the time of the announced shutdown, it was indicated that data was exportable, but on 2019-02-28 found that their Facebook login integration was down, login with a password wasn't working, requesting a new password wasn't working, and their support email address had been shut down and was bouncing back messages.

Other export methods may include:
 * Screen scraping
 * RSS feeds from library will give last 50 items if the user has a small library
 * http://www.citeulike.org/user/username has an export button that should allow users to export in a number of usable formats

The service allowed export of citations in a variety of formats including:
 * RIS - Export as RIS which can be imported into most citation managers
 * BibTeX - Export as BibTeX which can be imported into most citation/bibliography managers
 * PDF - Export formatted citations as PDF
 * RTF - Export formatted citations as RTF which can be imported into most word processors
 * Delicious - Export in format suitable for direct import into delicious.com.
 * Formatted Text - Export formatted citations as plain text

Shutdown
On 2019-02-19 the service announced that they would be shutting down on 2019-03-30 Tuesday 19 February, 17:37 CiteULike is closing down After nearly 15 years operating CiteULike, we’ve made the difficult decision to close the site. Unfortunately, the costs associated with providing it and the fact that none of us really has any time to put into the maintenance and development of the site mean that we have to call it a day. We know there are still a number of you out there who use the site regularly and we’re sure you’ll be disappointed but hope you’ll understand. You will be able to download your library until 30th March 2019 but after that it is likely that CiteULike will no longer be accessible. We will be refunding any Gold subscriptions pro rata that extend beyond that date. We wish you all success in your research and happiness in your life. The CiteULike team.

UI
Captured examples of some of the user interface that existed on the service prior to shutting down:
 * The service had a browser bookmarklet that allowed people to quickly bookmark pages and add them to their libraries
 * bookmark page: https://web.archive.org/web/20190302204949/http://www.citeulike.org/user/ChrisAldrich/article/9387574
 * Recommendations (aka favorite bookmarks): https://web.archive.org/web/20190228182533/http://www.citeulike.org/profile/ChrisAldrich/recommendations
 * Neighbors: https://web.archive.org/web/20190228182643/http://www.citeulike.org/neighbour/list/ChrisAldrich
 * Library: https://web.archive.org/web/20190228182801/http://www.citeulike.org/user/ChrisAldrich
 * Export: https://web.archive.org/web/20190228183204/http://www.citeulike.org/profile/ChrisAldrich/export
 * Profile: https://web.archive.org/web/20190228183954/http://www.citeulike.org/profile/ChrisAldrich
 * CiteGeist (popular posts in last n days): https://web.archive.org/web/20190228184925/http://www.citeulike.org/citegeist?days=7
 * Groups:
 * https://web.archive.org/web/20190228185028/http://www.citeulike.org/groups/browse
 * https://web.archive.org/web/20190228185303/http://www.citeulike.org/groupfunc/3124/home