Have you looked into how Patchwork approaches this issue? It's not quite equivalent to Mastodon, I think, but I am fascinated by their approach to 'mapping' social relationships. Would be very curious to hear what you or others with more experience on the matter think about it.

EDIT: to elaborate a bit. While I'm only just diving into these topics, I can't help but feel that a fundamental problem in many cases is that many problems of 'online social' have to do with a very unintuitive/inorganic implementation that just fundamentally doesn't work well for humans. What I found interesting about Patchwork is that it tries, to some extent, to resemble the way humans 'naturally' interact.

Now I'm not at all opposed to experiments in augmenting the way humans interact with others, but I can't help but feel that many of the things we've come up with that are popular went too far in not considering human nature (or intentionally messed with it). And perhaps a more constructive way forward is to come up with 'online social' tools that start with a more conservative resemblance to how humans have interacted for much of their history, and iterate from there.