I may have to do some backtracking because there are a few articles I skipped, but I thought this was interestingly noted in mention for the Twilight fandom:
The Twilight fandom accidentally made an empire. The fandom’s most popular fics—works like “The Office,” “Master of the Universe,” “Wide Awake,” “Emancipation Proclamation,” “Art after Five,” “The Submissive,” “The University of Edward Masen,” and a number of others—had readership that dwarfed most New York Times bestsellers. People outside the community began to see this readership and wanted in. It’s hard to get original fiction read, even for published writers. And the Twilight community offered more than eyeballs—it offered thousands of reviews, and publicity, and awards. In time, the Twilight fanfiction community grew to be less about the Twilight books and movies and more about the fanfiction, which in turn also became less about the books and movies and more about, well, romance writing in a mix of subgenres.
Also, I read Heidi8's article about Harry Potter and interestingly she mentioned this (quoted):
The Harry Potter fandom, which began in 1999 and shows no sign of ever ending, arrived in a perfect storm of radical new communication methods and interpersonal relations, which combined with the fantastic creativity of Harry Potter fans—creativity in writing, art, law, social networking, filmmaking, science, animation, humor, and a drive to change the world.
Kids and young adults who wrote Harry Potter fanfic in 2000 and 2002 and 2004 now have novels on New York Times bestseller lists.