Most Fanfic Writers Are Girls

Most Fanfic writers are female, and most are teenagers and young women.

Various theories abound as to why, but the cause is most likely related to the fact that most visual porn is male-oriented (see: any adult video store or site) while most written porn is female-oriented (see: your local bookstore's erotica section) and Fanfiction, like porn, trades heavily in Author Appeal.

This trope is extremely common, even in fandoms that presumably lean heavily male, such as Star Trek, Naruto, and Transformers.

This trope has been in effect since well before the internet made it much easier to proliferate fanfic. Studies of early Star Trek fanfiction showed as many as 90% of authors were female in the 1970s, even though at the time such stories could only be shared through fanzines or through sending self-addressed-stamped-envelopes to the authors and having them mail you a manuscript.

This phenomenon has been subjected to academic analysis by ethnographer Camille Bacon-Smith and MIT's Henry Jenkins. Jenkins suggests in Textual Poachers that fanfiction is a reaction on the part of a female audience trying to find their own pleasures in media that caters mostly to males.

Creates a paradox when faced with the trope There Are No Girls On the Internet.

Only examples where a character in the work writes a fanfic are to be added. Parodies and deconstructions should not be added.

Examples

Anime & Manga

Mahou Sensei Negima Haruna draws her own Doujinshi which is based of Mahou Shoujo Biblion and sells her works at Comiket.
Genshiken: Ogiue also draws Yaoi Doujinshi, much to the great trouble of her Editor boyfriend who is...not a fan. Well, Sasahara gets the appeal of yaoi to her... it's just more than a little awkward when she draws him and ships him with one of his best friends. Other than being featured himself, he's surprisingly accepting.
Harumi Fujiyoshi of Sayonara Zetsubou Sensei draws Doujinshi as well -- one of them about her Teacher Itoshiki-sensei. As well as a pairing of Apple × Pine when everyone else likes Pine × Apple (the characters in question are expies of Coco and Nuts). In the third series she is seen drawing a Gundam 00 Tieria (also voiced by Hiroshi Kamiya) Doujinshi.
Darker Than Black had an entire episode devoted to a character who writes in-universe slash fanfics about the protagonist.
Nobuko Yokokawa of Ojamajo Doremi writes stories loosely based on her friends and classmates with her best friend Aiko she idolizes as the Mary Sue protagonist in most of them. Later joined by the shy artist girl Miho Maruyama, who starts creating similar comics with her. With their stories involving romantic plots with their own self-insert characters along with Aiko's, it results in awkwardly heavy doses of Les Yay for a kids' anime.
Hikaru Amano of Martian Successor Nadesico portrays this in the English dub. She writes Self Insert Slash Fics.
Erika from Durarara openly discusses writing Slash Fics about her friends. She also ships Izaya/Shizuo.

Live-Action TV

Supernatural features a female slash fic writer as a recurring character, with serious stalking tendency. Supernatural has an entire in-universe slash fandom writing about the show's main characters.
In the season two Psych episode with the case at the school for smart kids, one of the (male) students says to Shawn:

Student: Any leads? I'm adapting this into a Nancy Drew fanfic.

Inverted with Glee; Sandy openly admits to writing fanfiction.
It's implied that Willow of Buffy the Vampire Slayer used to write Doogie Howser, M.D. fanfic. (This is probably a Shout Out to Neil Patrick Harris, the star of that show, who is a friend of Joss Whedon. And ironically, her eventual co-star on How I Met Your Mother.)
In Degrassi: The Next Generation, Clare starts writing Mary Sue fanfic based on a fictional vampire book series called Fortnight.
In an episode of Monk, Monk's self-proclaimed biggest fan asks him why doesn't he use his gun during a case. When he tells her that he doesn't carry one, she remembers that she made that detail up in a fanfic she wrote about him.

Video Games

In Dragon Age II, Isabela writes "friend fiction". We only get to hear a bit featuring Aveline and Donnic but according to Isabela, she does it for everyone.
- However, she does it only as a fan of Varric, who is also a professional writer, who uses his friends as thinly veiled "inspirations" all the time.
In the Fable III side quest, The Pen is Mightier, one of the books is entitled The Pangs of Sunset. It is essentially just an erotic novel featuring the Heros from the previous game. It is implied to contain both Slash and Femslash and is written by a woman named Ilona Pureheart

Web Comics

Garnet and Gure features this strip, wherein the largely female culture of fanfiction is explained.
Questionable Content features two female fanfic writers, Tai and Marigold; Tai's stuff is apparently good (she writes a raved about Harry Potter piece), Marigold's, not so much. After a near miss where lesbian Tai hits on straight Marigold, the latter seeks to make up for the awkwardness by posting a story to Tai wherein Hermione and Ginny "go all the way."
Aeris from VG Cats writes erotic fanfiction about Tidus and Auron, has dreams about Fox and Falco, and likes the idea of a hot molotov throbbing in Bill's hand.
Rose in Homestuck writes really awful wizard fanfic.
- Andrew once said that said fanfic is supposed to be Rose's subconscious retelling Hivebent.
- There's little evidence to suggest this was actually fanfiction. Only Dave ever called it fanfiction, and its status as a book series in post-scratch Earth suggests it's just a wizard story, not a fanfic.
- Additionally, the female troll UranianUmbra admits to writing fanfic about the Alpha Kids.
In Shortpacked, Amber writes a Web Serial Novel that is a pastiche of Twilight.

Web Original

Donut of Red vs. Blue mentions writing some Harry Potter fanfiction but well...
Tea is made this in Yu-gi-oh Abridged.
- She writes Harry Potter fics, and ships Harry/Hedwig, in case you were curious.
Similar to the Yu-gi-oh Abridged example, Twilight Sparkle is an avid fanfiction writer in Friendship Is Witchcraft... except she writes fanfic about her friends. Creepy.
The Nostalgia Chick is seen in a Flash Back apparently writing Lord of the Rings slashfic.

Western Animation

In an episode of Family Guy, Peter insists that Meg is going to write Ugly Betty fanfiction for a career.

Other

This Time article on fan fiction mentions this phenomenon.
An article by the Guardian also mentions this observation, linking directly to this very page.