… Peering inside the digital packets of data zipping across the Internet — in real time, for tens of thousands of users at once — was commercially impractical until recently. But the ceaseless march of processing power has made it feasible.Unsurprisingly, companies have been trying to turn this potential into profit. By tracking users’ Web habits this closely, they can gain a much more detailed picture of their interests — and then display precisely targeted, premium-priced ads.
http://www.washingtonpost.com/wp-dyn/content/article/2008/08/20/AR2008082003259.html