International sampling from mostly first world/developed nations and given the fact that everyone here has the resources/access for prolonged recreational internet usage, it's safe to say that most are from minimally working middle class backgrounds.

I'd say more than likely in the 90s - low 100s range; higher than most national averages.
I don't think there are really all that many legitimately stupid people on this site; just immature, which can just as easily cause similar deterioration of any attempt at intelligent discussion.