Earlier quoted context omitted.
This is what friend requests are for. I don't want unsolicited messages from people. It's bad enough for me on LinkedIn, where that's the whole point.
It isn’t always socially appropriate to friend request people, which is why I rarely do it (because I have no idea what the other person’s expectations are). If I’ve been introduced to a total stranger via Messenger, I don’t necessarily want to (nor should I have to) be their “friend” just to have them see my messages.
If I don't want them to be Facebook friends, then I'll give them my email.