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@peterkal @atomicpoet @fediversenews Google is your friend (until it's not)
https://xmpp.org/ . Open instant messaging protocol. There was a time you could use any compliant instant-messaging app to send (Facebook) "messenger" messages. So, some people built up a network of contacts, and used it WITHOUT logging in to the facebook app/web-page. Suddenly pidgin and other chat clients were unable to authenticate, and you HAD to use Facebook to receive/send messages.