I think Sandra's argument suggests that federation is really not such a great idea, since most users will still depend in intermediaries for their interactions, and the intermediaries may pull the rug from under users at any time. P2P networks shine here, because users register with the network, not with a specific server, so there's no one operator to pull the rug from under users, and no rug to be pulled. I've referred to federated systems as semi-centralized to make this point