I wonder if we’ll move back to small static online communities (as were popular 2 or so decades ago). PhpBB boards with tens of regulars all of whom you recognize.
Frankly it's too difficult to moderate at scale. And either way most users don't want to be part of the same big community as everyone else. They just want to be able to share content between their communities without a tonne of unnecessary friction or many different needless login credentials.
Twitter succeeded here because it allowed these communities to seamlessly blend into each other while allowing individual users to prune problematic individuals from their view of the community in a way that just wasn't feasible with other social media or forum platforms.
The fediverse (if the projects could fix their UX issues) look like they have the best shot of recreating this "small, discrete communities all intermixed" kind of environment. Hopefully however without a lot of the forced discovery and content rotating that Twitter currently pushes.
We've got to see the large scale problems in the last decade, but I think a lot of people either forgot, or were not alive for the small scale problems that existed back in the days of BBS's and early websites. The biggest issue with with most small communities is they tend to die quickly unless they have some unique bonding factor. For example a local RC car racing group. But that isn't generally interesting over a wide range of topics outside a few topics. So now you're looking at building multiple communities and managing them. But you'll find most people won't manage that well and they'll tend to gravitate to a few busier larger communities. But then those larger communities will grow much like the seed of a star via gravity, someone will know someone else that invites them and suddenly you'll have your own little facebook/twatter rife with all the same problems and split views on what is acceptable.
There are just a number of problems here that seem intractable when you have hundreds of millions/billions of people that want to be online and in communities of one kind or another, but an unending number of social problems that occur when you get large numbers of people gravitating towards interesting things.
You could get that tight nit group and maybe it kinda dies out for a year or two but something revitalized the community and it came back. With smaller, centralised communities this falls apart unfortunately since you have to rely on the host to maintain that service and if they shut it down, it's just gone. Federated services on the other hand have the advantage that as long as the participants continue to be part of that community, it can be revived on demand.
So in this sense, I think some of the major issues that prevented small communities from persisting and thriving have been solved. Of course like you mentioned the moderation, growth management, and UX aspects have yet to be solved but those have the advantage of being able to be largely independent of the core functionality and can be improved with time without breaking the world.
I also think the twitter approach to community formation works really well here. It has it's own issues but a lot of the more niche twitter communities tend to just fracture back into manageable micro-communities once they get bigger than a certain size. It's not as noticeable since what community you see depends on who you follow but it's 100% a thing that happens. I've personally seen it happen more times than I can count at this point.
Two solutions for intentionally keeping communities small and preventing low quality content are making joining "referral only" (like https://tildes.net/ for example) or requiring small fee entry (like https://forums.somethingawful.com/index.php) but each of those has their own problem. The prior has an issue of creating echo chambers as only "like minded individuals" can/will join while the latter can promote the exclusion of less well off individuals or those in regions with limited access to digital finance as well as posing a privacy concern unless some form of privacy preserving money is used.
Honestly I'm excited. Social Media from a technical perspective is finally getting interesting again.
Everyone will sit under their own vine
and under their own fig tree,
and no one will make them afraid,
for the Lord Almighty has spoken.
Part of the problem with being on a site like HN is we see everyone here bitching and groaning about how much 'big web' sucks and how we should get away with it, while missing that most of us here are also visiting that same big web with a few billion other people every day. I'll believe twitter/FB/etc is dying when I watch them turn off their servers for the last time.
No pictures and great moderation is basically the opposite of Instagram and I think exactly why real discussion takes place on here.