Since it is coming up again:
Mastodon isn't private.
Expect anything you toot to be visible to everyone, regardless of your settings for visibility.
Mastodon is built on a system that isn't appropriate for private conversation and the work arounds for private toots and the like are not in any way robust against someone just ignoring them.
@inmysocks perhaps a lot of folk here are way too young to remember CB radio being a popular hobby - there are are some parallels with this subculture (although today you have more features).
Anyone could join in and communicate on whatever frequencies were legal in their country (or take a chance with eluding the authorities) - but none of it was private, anyone else with a radio scanner and recording equipment could monitor the transmissions.
inmysocks Of Doom, Naked KSC
Alex@rtnVFRmedia Suffolk UK