IMO Facebook has an even better solution: simply don't let people send you unsolicited private messages unless they're a friend or a friend of a friend. Lots of email spam gets through my spam filters, but I've never been spammed through Facebook Messenger before (except maybe once or twice when a friend got their account compromised).

Things get much harder when you want to view public posts by strangers, but I imagine some kind of similar reputation-based system could still work.