There's a bit of nuance to that question. It's easy to get the user ID from a social network, but more difficult to determine if they are logged in. I'll help with the former, you'll have to determine the best way to check if they are logged in for your own site.
$id = JFBCFactory::usermap()->getJoomlaUserId(123, 'facebook');
If $id is null, the Joomla user with ID 123 hasn't mapped their account to Facebook. It's as simple as that.
There is no way that JFBConnect logs if the user that is current logged in did so through the social network. You can only tell that they are logged in and if they've linked their account to a social network, not what choice they used to login on this visit.
I hope that helps,
Alex