That depends on your threat model. All lemmy posts are publicly visible and can be scooped up by Farcebook, google et al. Discord is very definitely not properly private but all posts aren't public. They are undoubtedly doing the same thing FB does and selling a semi anonymised set of meta data about you, but the world doesn't have direct visibility
I know the three letter acronyms have access to everything I do, hidden or not, I don't like it but I don't see anyway around it.
I can however do my level best to keep FB, google, M$ out of my stuff to some extent