My Little Pony: Friendship is Magic! is probably the longest thread to have been created. It was hosted on MMO-Champion.com, hit 239,345 posts and 11,968 pages before it had to be locked cause of technical limitations. Another thread has been started in its place, My Little Pony - Friendship is Magic (part 2). As of 10-27-2011 you can still catch the original thread in MMO-Champion's search feature if you want to check out this part of internet history.
I went to the longest thread in the world today. It was pony-tastic!