I do not want my posts from anywhere on the Fediverse on FaceBook.
I have have seen people express worry over FaceBook posts showing up on the Fediverse. But, what about our posts showing up on FaceBook.
If Meta federates with the Fediverse, do my Mastodon posts (e.g.) show up on FaceBook?
Pretty much this, your post could still end up on search engine results, people could share your post through posting the URL on other sites, etc.
It just means there’s better connectivity and (hopefully, in the near future) better content serving. Also you don’t need to create an account for every single site (if you dont want to)