Have you ever been on a trip and found out later that a good friend happened to be in the same area at the same time, but neither of you realized it? Ban.jo is an app for Android and iPhone that uses multiple social networking sites to let you know when your friends are nearby and help you connect with them. According to Banjo:
Banjo gives you Social SUPERPOWERS! Banjo taps into the most powerful social networks: Facebook, Twitter, Foursquare, Instagram and more, to provide a real time view of what’s happening anyplace, anytime. Our powerful patent-pending ripple technology monitors the social graph in real time so you don’t have to. Banjo alerts you when friends are near so you never miss the chance to connect.
Banjo also has a blog.