but this assumption starts to break down when you look a little closer.
first, consider #ActivityPub C2S. why is there close to zero usage of this in #fediverse software? simple: it doesn't solve any needs for building a "network" "protocol".
now consider S2S. why are there zero compliant impls in fedi? because AP as specified doesn't address the needs of fedi. what does fedi need? well, i find it telling that the "real" reason AP was adopted was... to implement followers-only posts.
4/?
which is to say: the primary reason that #ActivityPub is used (to the extent you can say it is being used at all) in the #fediverse is mostly historical.
fedi grew out of a long line of open protocols, and before AP was adopted, it was at the point where people primarily used "activity streams" as their vocabulary and data model, stuffed into atom feeds. atom feeds don't do private posts unless you make an entirely new access-controlled feed, possibly with a token of some sort. hence, AS2.
5/?