The app collected data from FourSquare, showing local bars where women had checked in, and matched that with information from their Facebook profile, including photos and sometimes their dating status. The end result was that the app's users could see how many single women were in a particular nightspot, what they looked like and what their names were.

FourSquare blocked the app's use of its API, claiming it violated its privacy policies. That forced its developer, the Russian company i-Free, to from the App Store.

But i-Free didn't hack into people's accounts to get at the information, it only used what people had made freely available on their social networking sites. The incident shows how compiling such public information can make people uncomfortable when it's done in unexpected ways.

"When you see something so out of context with what you expect, it ends up being shocking," said Jules Polonetsky, the director of the Future of Privacy Forum. "I get that when I'm out in a big crowd, I'm not secret. But it's still seems bizarre if someone scans every face in the crowd and then somehow identifies it. It seems to push beyond the appropriate context."

The problem, to some extent, resides in a culture gap between developers, who think that if information is available, they can use it to innovate in any way they see fit, and users, who don't always understand how revealing their digital information can be, privacy advocates said.