Ryan McIntyre has an interesting viewpoint:
To me, the most profound aspect of Google’s pay-per-click model is the fact that it is powered by an auction marketplace that resembles eBay’s, only it is far more efficient and is sublimely frictionless, with Google collecting a much larger share of each transaction since they are not only the marketplace, but also the supplier and seller of the inventory. Even more impressive is that Google runs a real-time auction (which is really a hairy multivariate optimization problem that would impress even the brightest Wall Street quant) to serve advertising links every time someone searches on Google or views an AdSense-enabled page.
To put this in perspective, the NYSE and NASDAQ see average daily trading volumes in the low billions, while Google is certainly processing and delivering a comparable order of magnitude of advertising auction transactions every day given the number of searches performed daily coupled with the much greater number of page views generated by AdSense publishers.
