Do you *know* or assume to know? Legally speaking, you grant Facebook that right when you accept the terms of service, but the reality is that this right is not invoked unless it is involving a subpoena requesting that, otherwise they (and any company doing the same) would be in trouble. At work (one company that has similar terms of use), the only allowed access to private data requires legal approval and it is to produce data for Legal to review and, if necessary, to hand over to law enforcement/courts after a subpoena.
It goes beyond that. Not only do they monitor what you post, they monitor everything you type, even if you don't post it.
https://www.rt.com/usa/facebook-tracks-everything-written-posted-394/