Facebook intros new opt-in feature called Nearby Friends (for Android and iPhone)
Nearby Friends isn’t a feature that will be enabled by default. Users can turn it on (or off) at any time, and they can choose which friends to see. To detect a friend nearby, both you and him/her need to have the new feature enabled.
Facebook didn’t say if the new feature requires certain versions of Android and iOS (probably yes, though), Anyway, you can find out more about Nearby Friends at the source link below.
source: Facebook
Things that are NOT allowed: