Facebook has begun rolling out a new feature for its Android and iPhone userbase, whereby you can now find "Nearby Friends". At present a small group of people is currently using the feature and it will rolled out for US only in the coming weeks. Facebook taps into your GPS to get the exact location and then helps you to locate your friends which have this feature enabled.
Don't go all Snowden on it, the feature is optional. Both you and your friend needs to turn this on for it to work, so no shadow stalking. Users will also be able to delete their location history and in a way this is similar to Foursquare but without the need to check-in.
