Need suggestions from wordpress experts and geeks.

I am using wordpress as a CMS for my website. I have disabled wp login and infact removed the code which calls wp-comments box. Now visitors can comment on posts using fb-comments box only.

I want to install forum plugin to let user put their questions and discuss on them. But dont want to enable wordpress login/registration. Which is the best forum plugin that can satisfy my need. fb connect is mandatory for that forum plugin.

Suppose if I allow to login using wp then I would like to remove wp-login page and use login/registration as a widget. Is it possible?

Please suggest me whatever you think is the best solution. I would love if I can work on forum-plugin without enabling wp login.

what wordpress is using for cms and forum?