For almost all of its history, AOL's most important feature was email; even more subscribers used it than visited the chat rooms. We have already seen how email was able to induce a Skinneresque check-in habit even among the first scientists to use it. Perceiving straightaway email's habit-forming properties, Steve Case and his team applied themselves to encouraging the addiction among subscribers. For one thing, users could send an unlimited number of emails. Prodigy, by contrast, having been slow to offer email, foolishly decided to charge users who sent more than thirty per month. At AOL, email was sent early and often. In fact, the whole user interface centered on the application. Immediately after logging on, one heard a pleasant male voice saying: "You've got mail." With that appeared a whimsical picture of a postbox stuffed with letters, a virtual cornucopia of potential rewards in human connection.

Tim Wu, The Attention Merchants, loc. 715