I think this is what federation looks like when it works. Yeah, everyone uses Gmail, but it's also ok to use another service. It still works and delivers a pretty similar experience. Your messages make it out of the Gmail garden.
> Web sites were federated, but everyone being web master for their homepage was too difficult, so we get GeoCities, and Yahoo Clubs, and later, Squarespace and Wix, etc. Then blogs were decentralized, but everyone self hosting and authoring blogs was too much, so then we get Wordpress.com and Tumblr, and Twitter, and FB, etc. (even USENET eventually developed super-large hubs like uunet)
Geocities was one of many roughly equivalent services. It was sort of a social network, but it was mostly just free hosting. Anyone could still link to anywhere and being on geocities wasn't a requirement for being on the web.
Squarespace is a pretty good product and not what I would consider harmul centralization, because it's still interoperable with other websites and they don't prevent people from hosting elsewhere. The web is still federated.
Something different has happened with social media. Different social media services aren't interoperable. I can't message you on Twitter from Facebook, or tweet at you on LinkedIn. You must stay in their walled garden to interact with their users. They're at such a critical mass that almost everyone is in their garden.
It seems like federation is a happy middle ground between decentralization and centralized walled gardens.
Maybe something like Mastodon will some day become so popular that Twitter is forced to become part of the federation and compete on features and marketing instead of vendor lock-in.
With a federated version of any type of social media clone, eventually new entrants will prefer to join the federation instead of trying to make a new walled garden. It wasn't really an option for Hotmail to only let you interact with other Hotmail users, because the federation was already established.
There was tremendous demand for social media services, but no federation to provide those services when the demand hit and explosive growth happened, so now we have a centralization problem.