I still see three possible ways for Embrace, Extend, Extinguish.

The first is the Android way: while Android is FLOSS on paper, Google makes sure to cram as much important functionality as possible into their proprietary and closed Play Services blob. I have no intimate knowledge of ActivityPub, but I reckon it would be relatively easy to contribute a tailored modules wrapper under any required GPL licence, and then use that wrapper as a gateway to closed-source extensions.

The second is the Gmail way as described by Emi here: build a portal based on established standards that becomes hugely popular because of its ease of use and feature set, then start sneaking in cool non-standard features that only work inside your walled garden.

The third is the Microsoft Java way: build your closed-source clone from scratch and make it just incompatible enough for non-technical users to think that the original implementation is broken.

There's also nothing stopping Facebook or anybody else from just making their own clone of the software without copyright issues. If it talks the protocol the same way it will work.