Last year, Facebook created Safety Check, a tool specifically for "disasters and crises" that helps you see whether your friends and family in an affected area are safe. Today, Facebook turned on the tool in response to the horrible attacks in Paris. If you go to the Safety Check page, Facebook will list all of your Facebook friends in Paris who have checked in as safe. Furthermore, the tool will also notify you when other people on your friends list have checked in.
The flip side of this is that if you're currently in Paris or you're in an area of crisis in the future, you can let Facebook know you're safe, and it will inform your friends.
Our thoughts are with everyone in Paris tonight.