In terms of posting one's religious or political views, FB has no anonymity and although you might want to share your personal beliefs with some friends, you don't necessarily want to put that out to your boss who sent you a lame ass friend request, yet you feel obligated to accept it.
On a forum, I am Hyroglyphx, not my real name. There is a level of anonymity where we can simply share ideas without it affecting our day to day lives.
This is exactly the issue for me. There's no way I'd let all the stupid shit I say here get out to my friends, family, and coworkers.
I don't always debate, but when I do, I do it anonymously.
I think part of that has to do with floating ideas that I'm not vested in to see how they stand up to scrutiny, or taking a position I don't agree with to get better ideas on how to combat it.
Being anonymous allows me to not have to worry about people I know thinking I'm crazy.