At Cult of Mac, John Brownlee writes about Girls Around Me, a creepy app that exploited geolocation APIs to make it easy to stalk women.
These are all girls with publicly visible Facebook profiles who have checked into these locations recently using Foursquare. Girls Around Me then shows you a map where all the girls in your area trackable by Foursquare area. If there’s more than one girl at a location, you see the number of girls there in a red bubble. Click on that, and you can see pictures of all the girls who are at that location at any given time. The pictures you are seeing are their social network profile pictures.
See also Charlie Sorrel's guide to kill the Facebook and FourSquare features that enable apps like this.