Word of the Tub Gods: like Twitter, Mastodon sets your posting default to Public. If you only want your toots visible to other people on the instance, go to Preferences > Other, and under Posting Defaults set Posting Privacy to "Unlisted".
@tubgod Correction: mastodon has a few "listed" timelines like the local timeline (only tubplex) and the federated timeline (tubplex and everything tubplex has ever heard about).
* "Unlisted" means that a post will not show up in this or any other instance's "listed" timelines. It will however go to all followers and be visible if someone goes directly to your profile page
* "Followers Only" will not be visible in your profile page, it'll only go to your followers
* "Direct" is a DM or PM just like you'd expect
* "Public" is default, which is kind of bad, and operates as expected.
See also the github issue for the feature you actually want - https://github.com/tootsuite/mastodon/issues/861
which is already implemented by the massively popular glitch-social fork of the software ( https://glitch-soc.github.io/docs/ )
@rmsugarcandy @tubgod "federated feed" is just everything in the instance timeline, plus everything public that tubplex knows about. It learns about people when you follow them, so if tubbers haven't done many outgoing follows, or those followed people don't post public posts, then no updates go to tubplex that it can put in its fed feed.
Personally I think the solution is "federation networks", which doesn't exist, but the idea is that you could gift a specific other instance with your full public timeline, presumably because you have similar moderation and privacy policies.
Instead a solution we got were relays, where if you follow the relay it will repeat involuntarily-scraped public messages to you. I'd rather have an empty fed feed than enable those jerks though.
