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!
It’s called social media, the entire purpose of its existence is for other people to see what you post. This is true for Reddit, Twitter, literally any social media site. I’m not saying, well other social media is just as bad, I’m saying, this is inherently how social media works. If you’re expecting anything you post on any social media to remain private or be completely erased from existence when you delete it, you’re either stupid or hopelessly uniformed.
There are some sites where you can allow only people you’ve friended/followed can see your posts, but that is not the default setting and doesn’t prevent someone you’ve shared your content with from saving and distributing it.
Most social media sites ask at the very least for your phone number and birthday when signing up; Lemmy doesn’t, they don’t have any personal information other than an email address and only if you choose to add that for account recovery.
If this article is news to you, then so might this headline: Warning: when you drive your car from one place to another on public roads you can be seen by other people. Car users should consider this carefully before driving.