People often mention email as an example of federated communication, but the way email works in practice doesn't entirely live up to that ideal. Good luck getting your own self-hosted email server to send emails that actually reach anyone using a major email provider; they'll just be blocked as spam.

In practice, email is much less federated than it seems. A significant proportion of people are just using gmail. You probably don't have to include that many providers to cover a majority of people in the US.

I think federation has promise, but federation in itself is not a solution. Technical approaches do not address the more fundamental issue that, regardless of the mechanics of the system, big players will have more influence on its operation and evolution. Thus we will always need sociopolitical mechanisms to restrict big players.