Is the type of advertising that turned Google from just one more cool internet start-up without a business model into a corporate superstar too good to be true?

“Cost-per-click” advertising is one of those alluring ideas that make the internet so disruptive. By letting advertisers pay only when someone clicks on their message, and by targeting advertisements – either by linking them to the results of internet searches, or by fitting them to the subject of whatever someone happens to be reading online – it represents a huge leap forward in efficiency.