Sonar

Sonar is a mobile app that lets you know when friends and the friends of your friends are in your vicinity.

The app uses information from networks such as Facebook, Twitter, LinkedIn, and Foursquare and alerts you when people in your vicinity share friends, acting like a good host. The idea here is to encourage you to take advantage of being in the same place at the same time, and form new, meaningful relationships with other humans through the use of technology.