Facebook wants you to know when your friends are nearby, if they're cool with that

Facebook has announced an interesting new feature called Nearby Friends which does pretty much exactly what you think it does. User can optionally turn on broad proximity notifications, so you and your friends can get a notice when your friends are nearby. You can set specific friends and friends lists to get these notifications, and it only works if both parties have it enabled. The new system also allows you to share specific GPS coordinates for a limited amount of time with selected contacts.

source: androidcentral