A group at the University of Rochester built an algorithm that uses tweets to automatically identify the social relationships of active Twitter users based on their tweets' content and geo-tagged location. After identifying who your friends are, the algorithm then uses the data from their online activity to pinpoint your exact location, even if you yourself are not tweeting anything about the status of your current burritos, or glib observations about TV shows.
The idea behind Flap ("Friendship + Location Analysis and Prediction") is quite simple. Let's say that the algorithm detected that Mark, Joe, and Bob are best friends. If Mark tweets "partying with my two best buds," and Joe tweets "just found a rat in my pancake LOL!" the system can venture a guess that Bob, who hasn't tweeted anything, is probably getting regrettable drunk food with Mark and Joe at Denny's at 2 in the morning.
Later, it traces Bob next to a pile of dumpster-side cholestervom.
The actual algorithm is naturally much more complex and has been designed for noble goals, like predicting the spread of pandemics and such. And certainly no technology developed for noble ends has ever been misused -- just ask Nobel himself.