Cant remember what year it was but at some point FB removed each members personal email address and replaced it with a username@facebook.com address.
This cut off the possibility that members might try to contact other members via personal email, and instead redirected all their communications through Facebooks servers.
Facebook needed to see those communications because... why not?
It was ironically done for privacy. Users actual e-mails were shared with friends, and people had too many "friends", causing their e-mail to be leaked. After a bit of trial and error on hiding the e-mails from scraping, they eventually hid all e-mails by default and only exposed the @facebook.com emails. The original idea of people only having 'real' friends turned out to not work. You can't trust your Facebook "friends", which should be alarmingly clear for everyone by now.
At least with username@facebook.com, you could contact friends on facebook without actually logging into facebook.
Actually I'd like more social networks to be "federated" like that. ActivityPub is one such protocol, whereby users on different networks like Mastodon and PeerTube can communicate.