Facebook just announced a new feature for Android and iPhone users called Nearby Friends. This is an optional feature that "occasionally" notifies you when Facebook friends are nearby, allowing you to "get in touch with them and meet up".
Users can turn the Nearby Friends feature on (or off) at any time, and they can choose which friends to see. To detect a friend nearby, both you and your friend will need to have the new feature enabled. You can choose to share a particular location with a select group of friends for a set period of time. When the feature is enabled, the friend will be able to see exactly where you are on a map.
Nearby Friends can also be used when you are travelling, so your friends who have been to a location you are visiting will be able to give your recommendations on places to see or restaurants to dine in.
According to Facebook, Nearby Friends will be available in the US in "the coming weeks." as an update to Facebook for Android and Facebook for iOS. You can find out more about Nearby Friends at the source link below.
source - Facebook