MyBB Community Forums

How can I allow users to sign up for my site only using Facebook

Take a look here for facebook connect:

http://mybbmodding.net/forums/showthread.php?tid=180

And to make them able to login only with facebook, i guess you can simple do that by deleting the line of code in your header and just left the facebook login.

Please explain in detail

Install that plugin which i mentioned above.. After that you just have to delete line of code for normal login