ActivityPub, the protocol that powers the fediverse (including Mastodon – same caveats as the first two times, will be used interchangeably, deal with it) is not private. It is not even semi-…
TLDR: While Fediverse won't directly serve you ads, anonymous bad actors other than Meta can save, redistribute, and even dox you for any information you post here. Anything you post here can/will remain forever on some malicious instance that doesn't honor deletion requests.
So be careful!