If you want more fine-grained control over federation than Mastodon (which already has the basic knobs), Pleroma is more flexible in that regard.

It’s trivial to set up in a way that your content never leaves your server if you want, blanket or per-post.

Note I’m saying that content flows across servers and implementation, not that all of these share federated auth. You can not “sign in” on any other server than the one you registered with.

Some people have multiple accounts on multiple servers, some stick to one. fosstodon.org is a great one for floss and Linux talk if you don’t want to run your own.

BTW, check my HN profile if anyone wants to be friends :)