Facebook's 'Safety Check' allows you to see if any of your Facebook friends are in Paris. The safety check was implemented by Facebook last year to provide communication in the event of a natural disaster.
People in the affected areas must have location services turned on for it to work.
Here's how it works:
When the tool is activated after a natural disaster and if you're in the affected area, you'll receive a Facebook notification asking if you're safe.
Facebook determines your location by looking at the city you have listed in your profile, your last location if you've opted in to the Nearby Friends product, and the city where you are using the internet.