> I shudder to think about a system where random people could message me.

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.

Yeah, I have a phone number, and I don't pick it up if I don't recognize it. I don't know you, I probably don't want random communication from you.

I am pleased with the state of spam detection for things that implement it. Google does a good job, both on my phone (Pixel) and my email. Quora does not, and I resent it; it makes the site worse, so I turned it off.

If Facebook were letting strangers talk to me, I'd probably stop using it.

You just need a high enough barrier to entry or real consequences to deter malicious behavior. Spam & scams work because there's no downside; if they had to pay a fee to open an account every time they get banned for spam they'll move on very quickly.