what is the most famous forum in united states?for chatting

maybe it's called Bulletin Boards Systems (BBS).
i know in china there are many famous forums used for chatting in chinese.
its online population is always exceeded 10,000.some are alomst 100,000.
the content there are more professional than mylot.
i wonder if there are some like i said existed in united states.
for example professional forum for IT(information technology) people.
some one can tell me?
thank you very much.