It’s not like that at all.
When you walk out the door with a gun the manufacturer looses all control for what happens.
The content on Facebook resides on their premises and servers. They own it.
That is the key difference.
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)?
Regards,