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!
Other platforms just have the illusion of privacy
Do they? They state outright that they are going to abuse your privacy for profit. They have financial incentive to do so as it is their business model. Someone who is concerned about privacy deciding to use Facebook, Threads, Instagram, Twitter or whatever instead of Mastodon or Lemmy is completely ridiculous.