With open-source, I see it more a question of "How much will they freely give us?" Any proprietary extensions to ActivityPub is simply irrelevant to open-source projects, and would ultimately isolate their product from working with existing FOSS-powered infrastructure. If Facebook gives us a few bug fixes and dips into proprietary bullshit, I could that as a win. Who cares if they create some crazy proprietary version of ActivityPub? For most of the people that use things like Mastodon or Lemmy because they're privacy-conscious or dislike for-profit motivations in their software, they probably are willing to put up with a bit of a feature delta (as in fewer features on open-source software) to stay with software and hosters that aren't in it for the money.

For example, you could say that Autodesk creating the free tier of Fusion 360 with non-commercial restrictions has dampened the potential of individuals to freely license their work, but FreeCAD has happily coexisted for a long time. Sure, it's UX hell and is a bit rough around the edges, but holy shit is it amazing that I can license my work any damn way I please. That right there is worth infinitely more to me than some polished proprietary turd that tries to control me. I think many people will feel the same way about Lemmy or Mastodon, and Facebook's attempts to win over favor will be soiled by their proprietary nature. And heck, Lemmy itself is already impressively good for as new as it is. If Mastodon is even half as good, I'm sure the diehards will stay put.