If your email here and there are the same and the FB function is on and set up correctly, you can log in at FB and be logged on here. Or vice versa. It appears to me the feature is enabled but not set up yet.
You have to create an app at FB then it gives you a couple sets of "secret" numbers to put in the set up over here. That's from the admin side here. Then the member sets up thier account in thier usercp.
You can set it up also to let FB members bypass the normal sign up. Just a couple of items to deal with and you're signed up here. Spammers and trolls would love that.
To me it's just another useless dohickey VB
added. I'd would have rather they added some more local features. But to be straight up, I'd rather they stop playing around and fix all the bugs and make work right what we have now.