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.

@auerbach @bcrypt I think user mobility is the key to a lot of this. If people can easily vote with their feet and the above features are readily apparent, incentive will be to provide the features users want. Right now moving instances is a PITA.