There are a lot of never-ending threads I won't even open, and if you look at the last post column it always seems to be those that stay in that thread exclusively and don't really come out and play anywhere else. I think these threads must be chat rooms. One day I logged on and there were about nine of these game/chat threads all at the top of page one. Talk about fucking scary, and all the usernames in the last post column had numbers in them. I don't think I dare investigate.
I think some of it is simple human nature to be a part of something "everyone else" is doing. It might be easy to think that something must be pretty good if it has a 1,000 posts. People often reference previous threads and it sometimes feels like if you didn't read it, you're out of the joke.
For me, I'll scan the long threads that are about something worthwhile but most seen like games/flirting/chatroom stuff.