Facebook announced a new feature today that allows users that are in an area affected by a natural disaster to check in and let friends and family know they're safe. The feature uses whatever location information you've given Facebook.
This includes the city you've listed in your profile (whether or not you're currently there), Nearby Friends check-ins, or the city that you're connected to the internet with (as in the location of the mobile tower or Wi-Fi network you're presently connected to).
If there's a natural disaster reported in an area that matches one of these locations, Facebook will ask if you're OK. You can either say you're safe, which will post a status your friends can see, or say you're not in the affected area. There doesn't seem to be any action taken if you don't say you're safe (presumably because that would be a logistics nightmare and the job of emergency response teams), but it aims to give your friends and family some peace of mind if you are OK.
Introducing Safety Check [Facebook]