Maybe another explanation is that the site has been so helpful that people's problems have been solved. The pessimistic flip side, of course, would be that people gave up on the site. On a depression forum, for example, I wonder how many people stick around if their situation improves. Years ago, I left some sites for a long time, but nowadays I plan to stay even if I get better... seems like the least thing I can do to (try to) help others after they helped me (or even if they didn't).