But how would you cross reference it? From my usage of 4sq (which is limited, I only installed it maybe week ago.), when you check in to venue, and someone else is already there, you get "There are 3 persons already there!", with their avatars. You can tap on those avatars, and see some more info (John D., maybe some bio? I'm not sure, I do know that it doesn't give you their full name.), but I don't think that would be enough to cross match those. That's why I'm curious.
Off the top of my head, I'd say go look at the APIs for both FB and 4sq (try say user, friends, checkins, mayorship and location) and look at what is available for user PII and friends. Compare say username and do a little fuzzy matching on friend's names.
If say FB and 4sq share say more than x common friends, chances are you have the right mapping and you can build from that. Thrown in a Bayesian filter and pretty soon you will have a good model built up. Think of the 2 APIs as two sheets of paper with directed graphs on them. If you overlay them, the mapping becomes clear pretty quickly.
