It's not just friends that Messenger is useful for: Facebook Marketplace has become a big competitor to Craigslist, so leaving FB means you'll have a harder time selling your junk. Say what you want about Facebook, but at least my FBM ads don't get me a ton of messages from scammers.

It's been possible for quite some time to delete your Facebook account but continue to use Facebook messenger under those previous credentials, if you didn't know.

Do you have more information on this? My understanding was that you couldn't use the same credentials, instead registering a phone number for this.

All of the links I've conjured up are pointing to the same bit of information, will continue to hunt down more but I can say at least for my account I was able to fully delete my FB account, and login to the browser version of messenger.com using the same email/pass combo.

I've also been able to log into the Messenger mobile app without giving them a phone number (I was prompted, but not required at the time to enter a phone number). This all happened back in July of last year, so if the mechanism has changed since then, it's entirely possible you're correct that a phone number is now required instead of optional, as it was for me-and everyone else before the change and we just got 'grandfathered' in. Maybe?

I was aware you can deactivate your facebook account but keep using messenger. I wasn't aware that deleting your account was ever possible.

It’s a deliberate pig to delete but the pretence of deletion is there.

That's why science is useful. "Hey, I'd personally have a different result than what is seen in the paper" is not evidence that the paper is wrong.