> But a "network" of hundreds of thousands or millions of tiny instances, maybe paradoxically, doesn't have this same spam/trolling problem, any more than RSS did when we were all using Google Reader.
RSS/Google Reader was mostly a "pull" medium. Creators create content while consumers passively consume it but can't write back. This works but is fundamentally different from Twitter and the modern concept of social media. Furthermore, RSS offloads all the work to the clients, so RSS servers don't have to store nor process any state regardless of how many consumers there are (you can serve the same cached response to all of them).
If you want it to be like social media, you need people to also be able to "push" things (that others who don't directly follow you see) so people can reply and comment and do social-media-y things. This then not only opens the floodgates for spam, but also makes everything harder if every instance has to federate with tens of thousands of small ones instead of a few hundred like we currently have (the current system works fine with a "hosts"-like file manually curated by instance admins, but if instances start proliferating you'd need a DHT of sorts so they can all discover each other, and again need some way of vetting the quality of them so you don't get spam into it). This turns instances into hosts not just for the original content of the instance admin but also for the replies of people, again raising both server resource issues as well as issues relating to spam, illegal content and moderation in general.
> think of this like Blogger and Google Reader mashed together, with Twitter's surface-level user experience
Blogger could potentially pull it off because it's a closed system that has one consistent moderation policy. It doesn't have to broker or host unvetted third-party content. But blogging/publishing is fundamentally still a different medium to something like Twitter.
> People forget that the "blogosphere" worked pretty great before Twitter and Facebook wrecked it.
I think that the blogosphere and social media are fundamentally different mediums. Blogging is write-once, read-many, with minimal, mostly consumer-to-publisher interactions (comments on the blog itself). For a lot of people, social media also involves direct consumer-to-consumer communication, merely adjacent to the publisher's content but completely outside of their control. Your proposal does not address this use-case.