This is @deoxxa's overview of the various components of OStatus: https://www.fknsrs.biz/blog/don-statusnet-node-part-one-read-protocols.html
It's worth reading. IMO the component to think about is PubSubHubbub. Specifically, the hubs, which serve as notification intermediaries between publishers and subscribers.
Key notes:
1. Hubs do NOT need to be Twitter-like user homes.
2. Hubs are very capable of tracking user behavior, even if notifications are encrypted.
3. Hubs need to be reliable.
So the question: how to make hubs that are/have
* reliable
* trustworthy
* non-exploitative of users
* non-authoritarian (wrt censorship)
* proxy-able
* graceful failover
* graceful obsolescence
* potentially anonymous
* potentially transient
I think in the absence of these considerations, mastodon networks will, as others like @bcrypt have pointed out, tend to consolidate around a few large instances that will have too much trust & reliability burden placed on them.
I'm uncomfortable with hub-runners deciding which other instances they do and do not federate with. People can certainly run filtering services, but they should be independent of instances/hubs, not tied to them.
For similar reasons, I don't think hubs/instances should be thought of as communities with individual cultures, because you then turn the hub-runner into the de facto community leader.
Running a SMTP/NNTP/OSocial server shouldn't make you a doyenne. People get high off of that power.
auerbach
T0Y