Well, what really signed me on as a full member of the Internet (and as a fanfiction writer) was that '07-'08 Nickelodeon show, El Tigre: The Adventures of Manny Rivera. Yep, not really into it anymore...at all... But I did write some funny fanfics for it, and I enjoyed my life so much when I was into it.
And then I got into anime. Ever since ponies, I haven't really watched as much anime, but if I had to choose a couple that I was really into, it would probably be...hmm...Strawberry Panic! and Elfen Lied (yuri and gore; can you say "fanboy"?) Not particularly long series, but satisfactory for me nonetheless.
As you can tell, I'm not really into big, majorly established fandoms (I like it that way). Except My Little Pony, of course. This seems to be my current and only fandom, has been since January of 2012, and it's definitely a big one. I think the reason I've stuck with it for so long is because I'm older now, the show is still going, the fanbase is still growing, and the premise of it all is just so...interesting. Plus, I've met real people in real life who enjoy it as well, and, you know, that helps a lot.
Here are some...I guess you could say "minor" fandom-ish interests of mine:
Star Wars - I like the movies and Chewbacca/Wookiees are somewhat idols in my life
Green Day - I don't know if music can be considered a fandom, but as my favorite band, they made two of my favorite albums, American Idiot and 21st Century Breakdown, the latter of which I plan on writing an entire story around
Warriors - This was my main book series since 4th grade, and the longest continuing series I've read. I stopped after Omen of the Stars: Sign of the Moon, and I want to get back into reading it, but it will be so discombobulating to do so @_@
Naruto - Followed it valiantly with my brother back when it originally broadcasted on Toonami, when I was younger. Fell behind, didn't see some of the movies, lost track, lost interest, and haven't attempted to rewatch (have you seen how f***ing long that series and Shippuden are?!)
Fallout - I've only played Fallout 3, but I have been playing it for so long, and I still am not finished with it because I really take my time in that game, that it has to be one of the most satisfying video games I have ever played. I also have read Fallout: Equestria (haven't finished it, NO SPOILERS), and that is probably the most satisfying fanfiction...anything I have ever read. Plus, my friend and I talk about it and New Vegas a lot, and I have the honor of being a character in the NV fanfiction story he is writing. Lucky me
Halo/Red vs. Blue - Loved Halo ever since my brother and I got our Xbox 360 and Halo 3 back in Christmas 2007. I watched the Red vs. Blue series when I was younger, and very very recently--we're talking within the past few days here--I rewatched more of the series. It really is a funny and ingenious show. Caboose, Grif, and Lopez are my favorites.