When viewing a post, the version of Danbooru another site is using (1.18.0) says
Favorited by: <people>
I think that
Favorite of: <people>
reads more naturally. This fits in nicely with the "Add to favorites" command. I'm assuming that their wording isn't specific to their site, which is why I came here to mention this.
BTW, I tried seeing if this was discussed already, but got many matches for "favorited" unrelated to this post.