Among the various reasons for the dot-com crash was the fact that, over the late 1990s, AOL and everyone else had trouble proving that online ads necessarily did anything at all (hence AOL’s need to fabricate revenue). But with Google’s direct proof of clicks and tracking of customers, advertisers could finally see a direct link between their ads and eventual purchases. The beauty, once again, was that Google users were advertised to at a moment often very close to purchase. There was no need to prove the search engine could build brands—instead, it found users who were like travelers coming out of an airport, in a hurry, looking for a taxi, and there was the advertisement—“click.”

Tim Wu, The Attention Merchants, loc. 891