FanFiction.Net purged all forums on 25th November, 2010.
All unpinned forum threads that nobody posted in for at least 9 months have been deleted. The filter was moving up by forum creation date.
Despite the front page announcement made on the same day being: "To reduce clutter in many of the larger forums, we will begin to clear out forum threads with last posting date of older than 6 months. Pinned threads are excluded from the periodic purge." FFN decimated all forums with the last posting date over 9 months ago.
EDIT: On 26th November, 2010, FanFiction.Net changed the announcement to 9 months, solving the mystery why February 25 was the oldest date not deleted.
The result is that all historic value is lost, any useful guides, fun conversations and fandom information are lost. The process was automated, and it happened while America was sleeping (FFN is registered in America). Over a few hours from 7 AM GMT till 2 PM GMT forums made in 2005 and 2006 were purged, leaving their subscribers hanging. In fact, nobody will find out what happened till they wake up.
Why FFN decided to cull forums instead of poking trollfics is beyond me. If you want to save resources, reduce the amount of memory wasted by your servers, pay more attention to plagiarists, real-person fics and other things that can get you sued. What did forums have to go first is beyond me. Most importantly, why did they have to go overnight?
It's a sad day, very sad. With Fan History's closure just two days ago, the future of fan fiction does not look bright.
Edit of December 31st, 2010: FFN approved its claims on the purge being periodical, and did it again on Christmas Day.