Meta coming in and using ActivityPub means there will be rapid changes. It will be up to the open community to decide to go along for the ride or pick which parts of ActivityPub changes make sense to go along with. There is both good and bad in this. The good is Meta is a mature software company who regularly provides upstream changes (i.e. they contribute to the software community). I postulate that the changes they will make including things like design changes for better privacy, security, stability of ActivityPub protocol (if there is any) will be of tremendous benefit to the open community. They may also make design changes that benefit only Meta, and the fear there being the changes are detrimental to the open community. I don’t see that really happening for a few reasons. 1. Someone maintains the ActivityPub spec and retains oversight of changes (anyone know who that is?), 2. Any changes that detriment the open community can be dis-included from the open projects (Mastodon, lemmy, et al.), and community forks would be created. That is overall beneficial as long as it doesn’t fragment a finite resource (developers). 3. More users brought to the Fediverse mean more potential people to get involved in the open communities, e.g. if Meta starts pissing off their user base maybe it’s easier for them to jump ship to Mastodon.