By studying companies that collect "data transmission events" comScore analysts found that Yahoo! Inc., Google, Microsoft Corp., AOL and MySpace recorded 336 billion electronic transmission events in a month, keeping track of Web surfers' movements so they could place targeted ads in front of them.

This mostly invisible invasion of privacy hasn't provoked a large consumer reaction. Tracking people's movement on the Internet allows advertisers to "put dog food ads in front of people who have dogs," the chief executive of one ad placement agency said.

But, a recent study in California found 85 percent of adults didn't want their Web surfing activities tracked to help advertisers, the report said.

Television production is still limited to selling ads based on the size of the audience, while Internet companies can sell data about consumers and raise rates for advertising based on the ads' increased effectiveness.

"What is targeting in the long term?" Michael Galgon, Microsoft's chief advertising strategist, said to the Times. "You're getting content about things and messaging about things that are spot-on to who you are."