Using your publicly available profiles on Facebook, Foursquare and Twitter, etc, mobile app Sonar, shows you who, how many, and why particular people are relevant to you in a room.
Says founder Brett Martin, “It’s simple- you open up sonar and we tell you that the guy sitting across from you is facebook friends with your college roommate, the dude by the jukebox is a VC that you follow on twitter, and the cute girl by the bar also likes the Arcade Fire and Hemingway.”
This is a particularly difficult problem to solve as you almost never achieve hyper local density, anonymous strangers can be intimidating and most people are hesitant to adopt yet another app!