Do you think that Facebook is going to have only one instance? What I expect is that they will have a "main" instance for regular users, and then they will offer instance-as-a-service (infrastructure, hosting, customization and even admins) for corporations: a Boston Celtics instance, a McDonald's instance, etc., all federated with the main Facebook instance. I guess you can describe them as a walled garden, but once the Facebook-verse becomes mainstream, it will be a much larger garden that the few of us who decide to defederate them.

And then there is the issue of Chromium-ification. Once Facebook reaches a dominant position in (their walled garden portion of) the fediverse, they will implement their own features on top of ActivityPub, leaving us with an inferior version.