Twitter used to be always about who you choose to follow is what you see, or what tags you click on, it was opt-in and it was great. The popular feed wasn't your feed by default (via other peoples friends of friends which you have zero control over nor blue checkmarks being heavily promoted - which are usually the most tribalistic and FUDy).
Many subreddits have different sets of rules & restrictions. If these options were significantly expanded, could that ever meet the requirements?
Another interesting niche example is stackexchange.com, which isn’t a social network in the traditional sense (yet), but it has many different communities with different rules where a single identity, but variable set of privileges, follow you around.
Could these evolve into what you consider federated social networks, or are there more strict separation requirements (such as different login / identity providers among sub-communities)?
[edit: typo]
If a specific corporate site was coordinating the effort it would be shut down (eg, I think the Pirate Bay was knocked out of commission a few times, and blocked depending on the country). But it doesn't help because the actual data is highly distributed in a redundant fashion. Any piece of infrastructure to share torrents (DNS, websites, trackers, client software, information about where to look) can and has been targeted but it is too cheap to replace.
The key is the actual data - ie, the movie being torrented - is redundant and distributed. That property isn't present if one corporation controls all the data as is the case with Reddit, Facebook or StackExchange. It isn't technically prohibited from mirroring it all, but with bittorrent the peers provide the storage (which is the part that doesn't scale) and there is a nice cheap unit to work with - one movie is an irrelevant amount of data and static. Social data is a bit more dynamic - the attention of the speaker is important - and doesn't seem to work quite the same way.
Communication is a public utility. They shouldn’t need to be operated for profit.