Doesn't exist anymore (often enough).
> Just user-oriented effort instead of the constant barrage of difficult-to-setup tech demos.
Things like identity management and data storage make these barriers pretty deep. “I can delete my post and it basically won't be accessible anymore” (there can be physical exceptions so long as they're legibly exceptions to the social reality), “I don't have to think about how big my images are and can just post as much as I want”, “I can lose any of my own hardware and everything will still be there because it's in the cloud”, and “I can tell who my friends are based on common knowledge within my circles of their unique name which is easy to remember and meaningful” are all things that heavily constrain what you can do “on the inside”.
Mastodon has meanwhile managed to either do something right or get lucky wrt the path dependency of building structures where prosocial hosting behavior is convenient: a whole bunch of mostly-volunteer instances have sprung up, adopters have managed to make instance choice part of identity so that the domain-part isn't just a “meaningless extra thing to remember”, and federation remains reasonably strong; meanwhile, financial support for server costs has mostly leant toward the Patreon model, allowing a fraction of generous users to help support a bunch of free riders while not having to directly participate in administration. At the same time, despite Mastodon having almost exactly copied Twitter's model in terms of available user interactions, the zeitgeist has repeatedly suggested that users getting on board for the first time often had no idea what instances even could be, and had to have the very concept explained several different ways before it got real traction. Random instance death is also a problem that's tempering the mood nowadays, because keeping the server up requires enough motivation which sometimes runs out, and some instances have started having problems with media storage requirements, which, see above (though I'm told the internal architecture could use some optimization too).
There's something deeper in here surrounding the thorough conflation of type with instance in the popular side of the digital world; I feel as though something critical to the more literate concept of this didn't make it into the default folk model, such that only centralized services are legible. I have some hope that Mastodon and related ActivityPub-based federated services absorbing waves of people fleeing the abusive behavior of major social media (such as the recent Tumblr exodus) will make a dent in this and cause the appropriate concepts to reach critical cultural density.