In networked software, to be federated means users are able to send messages from one independent deployment to the other. In WordPress, this is achieved using linkbacks and feeds. So how would you define federation?

After Gravatars, I figured they'd try to disrupt Facebook with some kind of P2P plugin, like a Fidonet. But it never happened. Instead we got Disqus and IntenseDebate and bbPress...

RSS can be considered a federated protocol, maybe...if you stretch the definition.

With RSS, you can subscribe to feeds with any standard reader. Interestingly enough, YouTube channels all have RSS feeds, so you can subscribe to them without needing to do so via a Google account.