When George Reyes, Google's finance director, invoked the law of large numbers last week to explain why its revenue growth would slow, he briefly knocked 13 per cent off its market value. But Google and other search engines have more to fear from their reliance on a small number.

Search dominates advertising on the internet. The typical internet advertisement used to be a pop-up that got irritatingly in the way of what you were trying to read. It is now a snippet of text displayed on a search engine, linked to what you are searching for. Such advertisements account for 40 per cent of internet advertising revenues - double the percentage on display ads.