During the devastating attacks in Paris on Friday (13th November), Facebook enabled a service that allowed users to mark themselves and their friends as safe. This has been used in the past for natural disasters, and was rolled out for the first human disaster last week.
The service works by using location tools such as the home city you have listed and where you are connected to the internet to establish if you are near to where a disaster has happened. You will then be sent a Facebook notification to ask if you are safe. This will update your news feed, and notify you of friends who are also marked as safe.
Facebook CEO Mark Zuckerburg has since stated that this will now be used more widely for both natural and human disasters.