Probably thats the problem. More affluent(generalizing to educated) people and people who use better a network for technology, dont click on ads.
But of course, that's not even the point, because at the end of the day, you really care about maximizing user value, which for ads is frequency * (CPC or CPM). Now, obviously the CPC rates are higher for a BMW relative to a free iPod and the CPM rates are also higher if you know the audience is affluent, higher still if they are a fan of BMW on FB, higher still if you know their status is 'looking for a new car', etc. LinkedIn has fairly high CPM's from what I hear.
So is my conclusion that more affluent users are more valuable than less affluent ones? No, I don't know that, but that is my hypothesis, and I'd like to see some evidence pointing one way or the other (actual non-anecdotal evidence, and not just Myspace makes more than Facebook, since they run two very different ad systems and since we all know that correlation only goes so far).