Lately, one thing I've been reading, including on here, is how they're fans of a product, but "not in the fandom."

Let me explain what a fandom is.

To quote the basic definition of "fandom":

the fans of a sport or famous person

That's it. By being a fan of something, you are in the fandom by default.

You're a fan of a sports team? You're in the fandom of the team you follow and league operating under it.

You're a fan of anime/manga and the creative process of it in general (a.k.a., an otaku)? You're in the otaku/anime/manga fandom.

You're a fan of a celebrity? You're in the fandom of that celebrity.

You're a Mario fan? You're in the Mario fandom.

You're a Sonic fan? You're in the Sonic fandom.

You're a Whovian (fan of Dr. Who)? You're in the Whovian/Dr. Who fandom.

You're a brony (fan of FIM)? You're in the FIM/brony fandom.

You're a fan of a fanmade comic, video, or story? You're in the fandom within that fandom. I'm a fan of the well-done, in-progress brony comic, Twilight's First Day, so I'm in the TFD fandom within the brony fandom.

Well, you get the gist by now. By being a fan, you are in the fandom because you contribute to it by expanding its fanbase. Just because you're not active in the fandom mainly, if not at all, doesn't mean you're not in it. It means you're passively involved in the fandom.

So what does have to do with the short opener?

Not only is "I'm a fan, but not in the fandom" contradictory. It's factually incorrect. "I'm a fan, but not in the fandom" is also very pretentious.

By the way, by being on sites like this one and posting in FIM-related topics, you're contributing to the brony fandom in some way by expanding the forums and Poniverse's empire. (If you talk about other products in small doses, even from a nostalgic point of view, then you're contributing to those fandoms, too.)

The second the "I'm a fan, but not in the fandom" lie is quit being preached, the better.