«Google's real-time ad auction is the heart of their economic engine. Their stroke of genius, which gave them their seemingly insuperable lead over the other search engines in ad monetization, was to understand that selling the top ad position to the highest bidder was actually leaving money on the table. Given that advertisers only pay for clicks, Google realized that if they could project the likely click-rate on an ad, they could sell top position to the best combination of price and click rate. A $5 cpc ad clicked on 3x as often as a $10 cpc ad will make $15. By instrumenting, measuring, and responding to the click rate, Google made ad auctions smarter - through harnessing implicit user contribution.»