How does Facebook monitor such threats without monitoring all communication?
There is a way for you to report abuse on Facebook, as there is on FR. I've actually done that on FB when I witnessed over the top, abusive comments. Nothing was done. Facebook is making billions and needs to hire hundreds or thousands of monitors who will follow up on threats.
Close the accounts of people making threats.
It doesn't. Which is what they want---control, control, control.
“How does Facebook monitor such threats without monitoring all communication?”
It’s called “moderating”. They do it right here on FR. People who sign up on FB sign up of their own volition so if their communications are monitored they could easily cancel their membership if they liked. Where the problem comes in is when the Government monitors communications on a non-Government site. Anyone who thinks the things they post on FB, FR or any other site on the internet are private are either ignorant or new.