It’s not about the data they can collect. As long as we don’t use the Meta app or register in their instance we’re on the clear. The problem is giving them power.
I’m not an evil genius shithead like Zuck, but it could go this way:
They enter the Fediverse as the biggest instance.
They artificially slow down connections with other instances. That way, lots of users from smaller intances will migrate to the Meta one. Only the ones concerned about our privacy will remain in independent instances.
Once most of the userbase of the Fediverse is on their instance, they keep slowing it, or adding “features” only available in their app, effectively building a wall between them and the rest of instances.
Finally they defederate, leaving the rest of the fediverse weaker than it was.