John M. Simpson, the director of Consumer Watchdog's privacy project, said even if people understand what data they're sharing on social networks, they don't expect it to be "reconfigured so they can be hit upon."

"Just because something is technologically possible is no justification for necessarily doing it," he said.

"Many, many people have no idea the amount of information that they're sharing on Facebook," Simpson said. He faulted the social network's default privacy settings. "People sometimes go on and never realize the extent to which things can be seen. I think that's a worrisome thing. I think default settings are tremendously important."

But Polonetsky was reluctant to blame the social networks. He called FourSquare's privacy settings "reasonably intuitive." As for Facebook, he said, "After lots of pushing and pulling and squeezing, they've ended up with a much more usable set of controls."

FourSquare said the Girls Around Me app violated its privacy policy by "aggregating information across venues," and declined to comment further. Facebook did not respond to a request for comment.