If you want to know where your friends are at any given time, then Facebook is giving you an opportunity to find that out with a new feature they're rolling out for their iOS and Android apps. Nearby Friends lets you see what city or neighbourhood your friends are in, how long they have been there, and how many miles away from you they are.
This will only work though if you and your friend opt-in to use the feature. And if you are worried about privacy concerns, Facebook's answer is to let you choose which group gets to see where you are. It also shows only an approximate location unless you indicate your specific location and how long you plan to be there. Only at that time will you also see your friend's exact location. You can even enable push notifications to let you know if certain friends are in your area.
Nearby Friends can also show you friends who are travelling. The idea behind this is that you can suggest places for them to visit or even ask to join them if you're heading in that direction as well.
Source: CNet