Always remember, the people doing the bombing, and the racists and sectarians stirring away in the aftermath have a shared agenda. They both want to set people against each other on the basis of religion or "race".

Responding to those kind of posts just encourages them to keep coming back. If we could all resist the tempttion to reply to them, but instead could just scroll past them and ignore them, they'd drift away soon enough - and if they didn't, so what, it's just graffiti.