I recently did an analysis of the SPN newsletter (http://esorlehcar.livejournal.com/482808.html) (which collects and posts all stories from all the major comms, and is by far the most comprehensive record of stories in the LJ portion of the fandom) to see if the claims that gen was underrepresented was true. In the fourteen months I looked at, gen made up 40% of the stories posted, and that was in some ways artificially low, as a decent percentage of the stories in the "other" category were gen (any crossover, for instance, was marked "other").

I can't speak for other fandoms, but I do have the numbers for SPN, and the perception that there is "little genfic" in the fandom is just that... a perception.