If #Facebook/#meta joins the #Fediverse, it may incentivize more people to leave Facebook. Some people might be on the fence but stay because all the people they know are on Facebook. If they can leave and still keep their connections, it may be all the change they need.
Facebook must be aware of this. Can they block that from happening?
They don't have to. They will be the largest and by far the best funded instance on day 1. They can do whatever they want. Scrape the #fediverse for user data to sell? Easy. Create a nonprofit council that they control to manage the #activityub protocol? Easy. Incorporate proprietary code into the #meta instance? Who's going to stop them? Require other instances to comply or be defederated? Do you really have to ask?