Banjo pools shared information from several social networks -- including Twitter, Facebook, Instagram and FourSquare -- and sorts it by location. Users of the app receive push alerts when a friend from any of their social networks is nearby. They can also access the specific content shared by those friends.

In the past year, many have launched. These include the much-touted Highlight, , Sonar and Zaarly.

Banjo reached the million-user mark in just nine months, outpacing big social networking players including Facebook, Twitter and FourSquare. Of course, the number of users of a free app doesn't guarantee the app's long-term success. But Banjo's million users do suggest that at least some social discovery apps appeal to average users.

Banjo founder Damien Patton attributes his app's success to its inclusion of numerous social networks (though not Google+) and to what Patton says is a greater respect for users' privacy than other location-based apps have shown.

Banjo obtains location information from content that users have shared on social networks. It publishes the information only in ways that are consistent with the expectations users had when they shared it, according to Patton. For example, if a Facebook user shares a status update that includes a location identifier with his or her friends on Facebook, only those same friends will be able to see where that user is through Banjo. If the user allows friends of friends to see the status update on Facebook, then friends of friends will see his or her location in Banjo.