Every GameFAQs poll gives the Xbox and Xbox 360 more negative response. Like, "Do you have an Xbox?" always comes out to over 50% not having one, or uninterested so probably won't get ever one, etc.
The Nintendo systems do quite well, so going by the polls on GameFAQs you'd think that the GCN and PS2 were neck-and-neck (or at least a lot closer) with the Xbox as a distant third. Now, the PS2 is indeed ahead, but the GCN and Xbox are closer to each other in sales, and both quite distant from the PS2.
Which means, of course, that Nintendo fans will tend to be higher in proportion in any group of people that are online.
See the irony there?
Or, it could be because Nintendo gamers have to talk about games online with a lack of actually gaming while online. Or something.