We need to make a configuration guide or common support question for this.. it gets asked about once a month, and apologize for not making it more obvious. I've found a good post that explains pretty simply how to get the Login With Facebook on your Guests JomSocial page.. hopefully this helps:
www.sourcecoast.com/forums/topic?id=1704&p=1#p7224
Definitely post here (or in that post) if you run into any issues or have any questions!