The majority of the english speaking net is in the northern hemisphere anyway - with notable examples of southern hemisphere mostly being limited to australia and brasil, sure there are other net users south of the equator, but those are the two countries with the most presence online...

The reason why brasil's huge online population doesn't really count in these kind of things is because they largely keep to their own communities, often portugese-speaking.

So it being seasonal in appearance isn't really much of a surprise.

I'd also argue that geek-types (which most of us are) seem to be more prone to SAD.