To be honest, without the facebook integration of being able to talk to _your friends_, there are-as has been pointed out-far too many anonymous forums already.
1) how many people would sign up without a facebook account? 2) how many of them could you "convert" and sign-in again with facebook? (saying something like "ok..now, how about you use your facebook account to [blabla] with your friends on facebook?
I'd say people are more willing to give it a try and see how it looks without passing facebook credentials first... That's just my hint..