I think the first generation don't need to be as technically sophisticated as most people think. Get users in quickly (social logins), and let them pop out just as easily, even to the point of closing their profiles out until they get back. I think things like proximity detection or automatic social graph relevance algorithms are add-ons for later.
Good point. So what would be someone's motivation for "popping in"?
Meet someone new? Go bowling? What's the mentality behind a sign in?