I usually don't post in large threads because I read all of the comments and normally somebody said what I was going to say before I did, no point on saying the same thing all over again.
Most of the time when I comment on a large thread when it's gotten really long is because I made a comment when it wasn't long and somebody quoted me, though that doesn't happen often, some times I do find that rare time when nobody has said what I wanted to say and comment, if the thread is one of those when you have to say something about yourself, then I'll comment because at that rate, nobody is going to read it so I can say what I want to say without holding back.
If it's a forum game, well I don't post much on forum games because I normally start reading and never finish it and never post, there was a thread on Forum games that I did read around 500 pages of once, and it wasn't even interesting, I just wanted to post so I kept on reading and after that much I just gave up and posted at the end, and I hung around that thread for a while, but then I had to leave for a couple of months and when I came back there were too many pages between my last post and where it was, and I really had to read all of it so I could let myself post, but I didn't and have abandoned the thread, it's still going and I go and look at it from time to time but I no longer post on it.