Just because someone or something "resides" on something owned by someone else doesn't mean that that someone else should be held accountable.
I also own the house which I rent to a family. Am I responsible for the actions (or words) of that family?
Why should Facebook have to monitor the utterances of its posters - and decide whether or not said lawful utterances might constitute a "threat" (or be interpreted by someone else as threatening)?
Since Facebook handsomely profits from it they should be held liable. If it were a non-profit public service it might be a little different. Even the advertisers have some liability.
After all, if you own and profit from a whorehouse, you personally are not dipping your wick but you would go to jail anyway.