Like I said above, specifically for the “I want to socialize” use case of social media sites, there’s no upside to federation. It makes discovery harder, and a giant portion of what made Reddit so amazing was the random stumbling into things.
And yes, sure, federated systems can be made to more closely emulate such a centralized approach, but that’s why I said it that way: A centralized pool of social media content (for a given social media platform) is beneficial to the user, they can randomly stumble into topics and groups, and filter things down to what they desire.
In an ideal federated system, that is in turn exactly how the content would look for the user: They’d not even realize the content isn’t all on whatever instance they’re on, it’s fully transparent. Because that’s easier for the user. No matter how low the barrier to finding federated content is, there’s still no upside for the user having to take that step and go hunt for federated concept. From the perspective of the user, that is.
It’s not a big issue of course, but it does mean that by default, more users flock to where there are already more users.