@J12t it’s “on the fediverse” (not sure about capitalization); same as “on Facebook”, “on Twitter”, “on YouTube”, “on Patreon”, “on Wikipedia”, “on the web”, and so on

“in” is used to refer to a part of the content of a specific post, “someone mentioned that in a post/status/tweet/toot/…”

(In English, I don’t know German, tho I thought English was a Germanic language so I would have assumed it was the same until your post)