At least in the area of media studies, the term "media/medium" is still debated and there is no One True Definition. I suspect, it's the same in fandom.

To me, a media fandom centers around what offen falls colloquially under "multimedia": TV, computer, movie, radio, game consoles etc. Thus, any fandom deriving from a source seen/heard/played on these machines is, in my view of things, a media fandom. Comics, manga, manwha and so on are a bit of a hybrid in this system, as they often are closely tied to a TV show.