I agree there are definitely some circumstances in which it is a positive thing, but I feel this function that Facebook has of allowing people to mark themselves "safe" seems in some way to equate the experience of simply being in Greater London when a terrorist attack happens with the experience of those who are surviving in places which have ongoing mass atrocities (like Aleppo) where the language Facebook uses would be more appropriate.
If you know someone who is one of the 9 million people who live in London but they don't live, work and only occasionally go to the specific place where the attack happens, it is very unlikely they are one of the 7 killed and 50 injured.
I feel that it first of all undermines the attention that the families, people and aid workers who are really suffering as a result of the attack should be getting, by turning the focus to unrelated people, many of whom live up to 10 miles away from the attack. And that secondly it also gives people a false perception of the place and circumstances they live in by them being given the drama of having to declare themselves, or seeing their friends on Facebook declaring themselves "safe" or not.
I grew up in a war zone. Social media didn't exist at the time, so you didn't know if someone had been killed or injured and you never had an accurate idea about what was happening and where. But if it had existed, the state of war was so chronic and people were so hardened to it that you would've been laughed at for marking yourself safe if a bomb had gone off in a village a mile away, let alone a place you never went.
I know that terrorism is the same-level horrific whether it is a one off, ongoing or in any country of the world, but I feel Facebook is yet again warping our perception of ourselves and others by making a terrorist attack another chance for narcissism in a small way.