According to a poll of readers by the polling company Harris, here is how current graphic novel reading breaks down;
Amongst those who say they have read one book or any type in the past year, 11% have read a graphic novel during that time. However it biases young and male. 18% of 18-33 year olds have read a graphic novel, while only 4% of over 65s did. And despite the inroads of manga, 15% of males have read a graphic novel but only 8% of females.
The poll has been weighted to more accurately reflect the population demographic. So it's also worth noting that however you look at it, this is a major increase on what might have been expected ten or twenty years ago. The graphic novel has made significant inroads into the mainstream American population. And while no whee near the likes of France, Japan, Spain or Italy, it's a significant step towards an acceptance in the US of the Ninth Art as a valid form of entertainment.