Do you not have a phone number that anyone can call? Or an e-mail address that anyone can send to? Or have you used a platform like IRC that allows users anyone to send you direct messages?
> Glad you enjoyed Facebook before everybody got to enjoy Facebook, but most people never saw it like that
Facebook messenger isn't overrun by spammers. I've only used the messenger a handful of times but IIRC it wasn't hard to tell the difference between messages from friends and requests from people I wasn't friends with.
Spam detection also isn't terribly difficult at scale. Spammers need to message thousands or more accounts to even have a chance at converting someone, which is so far away from the normal use patterns of a real user that it's easy to flag.