This is what you get:

Brussels Explosions

FACEBOOK SAFETY CHECK

Quickly find and connect with friends in the area. Mark them safe if you know that they're OK.

Are you in the affected area?

Yes, let my friends know.

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Basically if you are in Brussels you let Facebook know you are ok. And if you have friends who knows you're in Brussels, they can check if you are ok. Rather then the poor Brussels person getting lots of messages from people asking if he is ok, they can check once.

If you live in that area you login and mark yourself as Safe, so everybody interested can see that instead of you receiving countless questions.

Also, I received a push notification on my Android that "Friend was marked as safe" from someone I know who lives there.

Facebook asks all its users that are close to a disaster if they are okay and alert their network.

It is basically just a fancy way for people to update their status following news of a disaster.