Email is my go-to example of all the problems of federation. In theory any person can stand up their own server and interact with everyone else. In practice, there's been so much abuse over the decades that it takes a staggering investment in time/energy/money/expertise to do so. Enough that's it's completely beyond the reach of the vast majority of people.
This problem is so bad that's it's driven a quiet de facto re-centralization of email.
Usenet is another example of the problems with 0 moderation and federation. Sure, it distributed amazingly (many ISPs mirrored usenet data), but once spammers started spamming, it quickly died as a discourse medium.
Those with concerns over censorship need to face the fact that we've previously had 100% censorship free platforms and nobody uses them anymore. We dumped those platforms because of the lack of censoring, not in spite of it.
Heck, HN is moderated as well, I don't think most of us would be here if it was unmoderated.
There is a major difference between censorship and moderation though that is frequently conflated.
Censorship is blocking/removing speech as judged by its content.
Moderation is warning/removing participants based on their behaviour.
They're not mutually exclusive of course, but it's frustrating to see moderation projected as Big Brother, when it's actually an extremely useful and necessary too for civil discourse. Moderation can be very effective without making any decisions about speech content.
The line between moderation and censorship is rather subjective to the moderator's own biases, don't you think? I'm not saying we should be free of moderation, just that it's very difficult to keep it completely objective, especially if there's any politics involved.
Why the downvotes? Was my question difficult to understand?
How does spam work on a social-media-like system where you have to wilfully follow someone to start getting their feed? Do you volontarily subscribe to receive spam?
You are assuming integrity and good-faith participation of all systems involved. This is not a reasonable assumption in the context of service abuse. Account compromise and host compromise spring to mind as avenues for spam. All it would take is popping a poorly configured server and then spam can be injected into the feeds you've chosen to follow.
Usenet servers had admins and tools to deal with spam: cancel messages and in extreme cases the UDP. If you were on a decently run server it wasn't any worse than Reddit or HN even.
Email is still vastly better than just about any other online communication platform we have devised since. Even with re-centralization, there's still plenty of small email providers - and the crucial part is that they all interoperate with GMail, Outlook.com, and other big brands. And for those who absolutely must run their own server, they can do that, even if it takes a lot of effort.
Is it bad that one email provider has lots of the market? It doesn't mean there's still not a long tail of alternatives for anyone to use if they feel like it. We haven't "standardized" on Gmail until no one else is using anything else, and we're a hell of a long way from that. Gmail has 30% of the market.
If the major advantage of federation is the ability of small players to function on an event footing, yet a handful of large players control the vast majority of the market, it calls into question how real this advantage is.
I don't think that's that major advantage at all. The major advantage is that there's many players. If the cost of entry is non-zero, maybe that's not ideal, but so be it. Food carts aren't free, but I don't see anyone saying, "Welp, screw it then, we might as well just all eat at McDonalds because the whole system of restaurant federation is useless."
This problem is so bad that's it's driven a quiet de facto re-centralization of email.