Facebook Safety Check - at first, it sounds like a privacy feature of sorts, but it's far from that. After an earthquake and tsunami hit Japan in 2011, a team of Facebook engineers worked on a system that allowed friends and family to check in with their loved ones and make sure that they were doing alright. Now, Facebook is releasing this feature to everyone.
If you happen to be involved in a natural disaster - or happen to be near one, Facebook will push a notification to you asking you to let your friends know you're safe. To determine your location, Facebook pulls some information from your profile - your current location. If they can't find it, because it's not listed, it'll also check where you last logged in from.
Here's a short video from Facebook to show how it's used: