Imagine being able to post only to Alice, Bob and Carol and nobody else ever laying their eyes on the post. Not in the Fediverse, not outside the Fediverse.
Imagine only Alice, Bob and Carol being able to reply to your posts, but all three being able to see and reply to each other's replies.
Imagine being able to define groups of connections with which you can do the above.
Sounds like utopian science-fiction. Is reality.
Hubzilla (official website), a Friendica fork by Friendica's own creator, offers literally what I've described above. It has since 2012, almost four years longer than Mastodon has been around.
If you want something more lightweight with not quite such a steep learning curve, there's also (streams) (code repository from 2021 from the same creator, the result of a whole series of forks. Similar advanced and fine-grained permissions system, but somewhat easier to use.