Griz,
Just saw your post on the Joomla forums about this. You explain there that you're looking to prevent bot registrations.
I'd highly, highly recommend looking into Registration Validator. It's a paid (under $20, I think) extension which will check emails against stopforumspam, spamhaus, and a few other bot checker sites. In early June, we got added to some spam SEO list and began getting about 100 registrations per day. I installed Reg Validator (on a recommendation from another Joomla user), and it took care of the problem. We get just a handful of spam reg's per week now, and I report those back to stopforumspam.com every time to do my part.
I get nothing out of you using them, but again, they've worked great for us. They don't integrate with JFBConnect (though we can tell you how), but all of our spam user's were filling out the normal Joomla registration, not coming through Facebook. I'm assuming you're seeing the same.
For more information:
extensions.joomla.org/extensions/access-...authentication/10802
Good luck,
Alex