I don't understand this thread...
Facebook is a public website, why would it be weird for anyone to message anyone on there?
I believe facebook has a block/report feature where you can eliminate people from viewing and interacting with your profile.
You need to take yourself out of devil's advocate and debate club mode if you can't see what the issue is here.
When somebody starts trying to move beyong this and come into your real life uninvited and on some harmful mess that's something to be pissed about that is a potentially big deal.