And that, I believe, is the real reason #Meta is implementing #activitypub: So that they can cross-post across all their apps, such as from #Facebook to #Threads, without running afoul of antitrust law specifically in the EU.
@Sarahp here has a piece on that cross-posting, although she does not mention ActivityPub. (Which they may or may not use for that purpose between their own apps .. whether they do is not really important)
This occurred to me when I opened up Instagram in a web browser the other day, and the Threads logo showed up all lonesome in the bottom left corner. One could very easily imagine to have Facebook and Whatsapp logos right next to it.