If you want to let your Facebook friends know that you're okay, you can visit Facebook's "Safety Check for Paris Terror Attacks" page. If you mark yourself as safe, friends will receive a notification.
Facebook prompts individuals to use the feature if location data from a phone or other device indicates they're in the area, according to the company's Safety Check page.
The Safety Check page also displays a list of friends who are in Paris, so you can see at a glance if you know anyone who might be in the area affected by the attacks.
According to Forbes, Facebook began developing the safety check feature after seeing how people used social media in the wake of the 2011 earthquake and tsunami in Japan.