[alphaBetaGamma]

I don’t understand this argument. Why would advertisers only bid if there is enough search volume? For each venue you advertize on you have a fixed cost that is the cost of setting up and maintaining your ad campaign on that platform. So there is an incentive to use high search volume sites, but this hardly qualifying as a natural monopoly.

Or am I missing something?

[prasunsen]

I don't think you are missing anything. There are advertisers who prefer other networks like Chitika, Adbrite, Clicksor etc because of the lower competition/lower click prices there (and not only).