Chilldoubt
Facebook are caught between a rock and a hard place here.
First of all, they would have to implement oversight scanning software and apply it using different protocos dependent on the local laws on privacy.
Then this software would have to be intelligent enough to differentiate between bravado and intent, e.g.:
I swear to God, the next time a Chugger stops me on Oxford Street, I'm going to kill him!
Then, if FaceBook identifies a pool of potential threats, they would have to contact the security services. Once this leaks out, FaceBook would have to hold it's hands up and say, "Not only do we sell your info to advertisers, but we are also in collusion with the state".
This would absolutely kill FB in places which have regimes which aren't as liberal as our own.
I suspect that FB will resist any attempts to make them accountable for this tragedy.
There are two people responsible for this - and happily the next time we see them is when their corpses are being transported from jail to unmarked graves.