QuantCast puts stackoverflow.com at about 3.0 million uniques per month. Given that they have such a targeted audience (all of us are interested in programming) I would think they could charge juicy rates for advertising. I would guess that they are getting $7.50 CPM (cost per thousand impressions) which would put their monthly revenue at $22.5k, annual revenue $270k. This is completely a guess and almost certainly wrong.
WebTrafficAgents guesses that stackoverflow.com is making around $320k/year.
