Fediverse

> make a new messenger using a niche protocol > new users choose your messenger because it is objectively the best after you dumped unreasonable amounts of cash into it > userbase grows, in large parts because the small messenger is interoperable so you can say "hey, if other company wanted to they could just implement [protocol] for you, we are already doing that" > once userbase reaches critical mass, pull the plug on the protocol > users with long chat histories and contact books are now more or less stuck on your platform whether they like it or not because getting people to switch suddenly means two messengers instead of one for them, not a good proposition to make.

XMPP did take off while it was in Messenger, Facebook decided to kill it with its superior reach because it was a step-ladder rather than something actually useful to them. Facebook will absolutely use the Mastodon interoperability as a marketing trick "Hey guys, if you have friends that don't like threads they can use another platform and still talk with you". They'll use it to distinguish threads from twitter until they feel like they don't need it anymore. Then they'll find some sort of technical excuse and pull the plug on ActivityPub support.