Pretty much who you’d guess:
According to FFN Research, the average user of FanFiction.Net in 2010 was a 15.8-year-old girl from the United States who didn’t write fan fiction herself. Not to say that 45-year-old mothers and adolescent boys don’t also read it, or that fan fiction is only written in English; but the odds are not good. And with a community that is 80 per cent teenage and 80 per cent female, with three-quarters signing in from Britain or one of its former colonies, can it be a surprise that the Harry Potter books have such a dedicated following?
Update from a few readers:
While I don’t doubt that the majority of fan fiction readers are teen-aged girls, the flawed methodology of the referenced study seriously undermines its conclusions.
The web site doesn’t require registered users to provides age/gender information, and the analysis is based solely on the small minority (10% or less) of users that voluntarily offer that data. It is not unreasonable to believe that younger facebook-generation readers are more willing to reveal their ages, or that boys might be less likely to fess up to reading fan fiction than girls. And while there are three times as many Harry Potter stories on the site than the second-most popular fandom (Twilight), the study was based on readers who registered in 2010 … after the peak of Pottermania. Those signing up in 2010 were far more likely interested in mindless-stories featuring angsty vampires.
As a longtime author and longer-time reader of fanfiction, I would submit that those statistics don’t show you who reads fanfiction so much as it shows you who reads fanfiction as a registered user at fanfiction.net. FF.net, despite its size and visibility, has had for years the (apparently well-deserved) reputation amongst the fanfic community of being entirely given over to giggling sixteen-year-old girls. Fanfiction.net is merely one of many archives, along with the collections at livejournal.com, on tumblr, at archiveofourown.org (the home site of the Organization for Transformative Works) and at innumerable fandom-specific sites.
And, honestly, I would suspect that getting any kind of accurate data on the true nature of the readership would be tricky: fanfiction is still Not Entirely Respectable, as hobbies go. I’ve been a fanfic author for years – and am proud of what I’ve written – but as far as my family and meatspace friends are concerned, I am so far in the fanfic-writing closet that I can see Narnia from here.