Facebook used XMPP to entice users to befriend Facebook users, then when they had enough market share they killed support for XMPP, forcing everyone who hadn't yet joined facebook to join or lose their contacts who were on Facebook. There used to be times where you could just use Trillian to talk to people on multiple platforms (including MSN and Facebook) at the same time, and that died off as the walls grew taller.
ActivityPub is a competitor to Facebooks walled garden. I would not doubt they would try to try and cannibalize it.