I feel a little lame quoting myself, but I was just having this discussion elsewhere so I'm just going to copy/paste my thoughts rather then thinking of a different way to say it this time.
Say you have 10 servers. 7 are Lemmy, 3 are kbin. Great, each admin has control over those servers. Then you have Meta. They'll run 1 huge server. When the 10 other servers enable Federation, Meta now has 10 servers of content that isn't even on their own platform that they can sell. Your data will literally exist on the Meta server because your data is not contained within your instance/platform once it's Federated. Meta can then harvest the entire Fediverse for data like this. It's like an absolute wet dream for them. They don't even have to coax people to use their own platform!
If your instance has defederated from Meta, but is federated with an instance that does federate with Meta, then Meta still has access to all your data through that mutual server. So not only would people have to defederate from Meta, they'd have to defederate with anyone who does federate with Meta. If everyone isn't on board with this, it'll cause a huge fracture to form.
Make no mistake: Meta wants to sell your data. They know all it takes is one server to federate with them and they've unlocked the entire fediverse to be harvested. I would not be shocked to see large amounts of cash flowing in exchange for federation rights.
Meta must be defederated the second they so much as dip a toe into the Fediverse or everything you've ever done, or do, on any ActivityHub platform will be scooped up and sold.
I'll just add that Meta will state that anything on their server is their property, and Federation will put your data directly on their server, even if you're not a member of their platform.