Nitpick: Decentralized and federated aren’t the same thing when referring to computer systems. Mastodon is both because anyone can run their own instance and have it interact with everyone else (decentralized), and because it’s built as an extension of a protocol that’s interoperable with non-Mastodon systems (federated).
Twitter, conversely, is centralized… but it’s also federated in the sense that it can serve as a federated identity provider. Ditto Facebook. It’s just that the social media components of those systems aren’t federated.
Email is an interesting case. Normally I’d argue that’s its decentralized but not federated… but I could potentially be swayed by an argument that it’s federated in the same context Mastodon is (i.e. is made up of separate yet interoperable implementations of an open standard).