Simple solution: have a "ask X if they're safe" button that friends/family can press. If someone presses it, that person gets a push to mark themselves safe. They don't get a push otherwise.
That will have the flip side of family members getting extra worried if not marked safe, because Facebook wasn't checked for example.
I'm guessing most family members are most likely not going to ask the question on facebook instead of just call their loved ones or send a text or email. My personal line of thinking is (maybe check facebook if they're super active to see if they said they're ok). Send a text or email if I'm a little worried. Call if I'm very worried. After a text or call I wouldn't go back and push the ask button on FB.
You always had that though. And you can't automatically answer it for users from device activity because that would violate someone's privacy.