That's true of pretty much any system, though: The Iron Law of Oligarchy[0] always applies.
How is that anything like reinventing a monolithic social network like Facebook?
As a separate analogy, consider a server for a MMO. MMO servers live and die based off the usercount, and lower count servers are considered less desirable for other incoming users. So what ends up happening is that players congregate around the server that has the most users, which in turn drives up costs for said server requiring them to take in more money (through donations or ads or subscriptions) and soon they become the standard for other servers to follow.
Even if other servers end up hosting, few reach the same numbers because most new users are instead redirected or drawn to existing servers with a high user count. So we're back to stage 1 where the fediverse hasn't really solved anything at all. Said competitor may then choose to instead jump off the fediverse and take their users with them (because you become associated with certain other negative groups which are bad for your image), the fediverse starves because your average user doesn't care and then we have Facebook again.
As a counter example, if the behavior you described really did happen in federated systems then why hasn't it happened to email?
Congratulations, we reinvented Facebook.
Anything with any hierarchy in its namespace will follow this path. Only pure P2P has a chance of avoiding it, and pure P2P is very hard to build and scale.
It also faces a serious protocol ossification challenge with no good solution (yet). Federated protocols face it too, but less as there are fewer cats to herd.