Mastodon doesn't fill your timeline with content from other instances, by default. If 100 million Facebook users federated with Mastodon overnight, 0 organic Meta-related posts would make it into my timeline. Only content reposted from Meta by people I follow would show up.
Defederated or not, Meta will have pretty much zero bearing on the instances outside their bubble.
Defederated or not, Meta will have pretty much zero bearing on the instances outside their bubble.