"[NebuAd is] building profiles," Harris said. "I think it's pseudonymous. It can't be entirely anonymous."

Harris, as well as officials from Google and Microsoft and some Democratic senators, called on the U.S. Congress to pass comprehensive privacy legislation that would govern how personal information can be used online. Voluntary rules set up by the online advertising industry and recommended privacy practices from the U.S. Federal Trade Commission aren't enough, Harris said.

Others at the hearing questioned the need for new privacy laws. Targeted advertising supports Internet content, and Internet users have differing levels of privacy needs, depending on what Web service they're using, said Clyde Wayne Crews Jr., vice president for policy at the Competitive Enterprise Institute, a conservative think tank.

"Privacy is not a thing to legislate, it's a relationship expressed in countless ways," Crews said. Legislation would be complex and would likely not keep up with technological advances, he added.

In addition, Internet users understand that the Web has privacy limitations, he added. "If privacy is what you want, the Internet is probably not for you," he said.

Senator Jim DeMint, a South Carolina Republican, suggested that competition, and not legislation, will solve most privacy issues online. "In some ways, we've got a solution in search of a problem, as the industry moves very quickly to cut off these problems before they occur," he said.