[Dave]

Hmm I think you can do this in the register_addmember_process hook and by doing something like:

PHP Code:

if(!is_facebookenabled() || (!vB_Facebook::instance()->userIsLoggedIn())){
   standard_error(fetch_error('your_only_fb_registrations_phrase'));
} 


There may be better ways, but this came up when I looked at the register.php code.

My personal opinion about the idea of only allowing FB registrations is kind of stupid, you will receive less registrations since forums are kinda meant to post things under an "anonymous" name.

You could just make it so that only people who made a forum account with their FB account can post in certain forums/threads.