Websites defend consumer-tracking tools

Several of the firms detailed corporate privacy policies and opt-out procedures in their responses, but Markey said such policies are often complicated and difficult to navigate.

"For example, a single website may have business relationships with a dozen or more third-party data firms that display advertisements on its site. A consumer may have to visit each of these sites, consulting its privacy policy and clicking through to opt-out, if such an option is provided," Markey added. "In some cases, a list of all third-party affiliates is not readily accessible, keeping consumers in the dark."

AT&T chief privacy officer Robert Quinn said his firm consolidated 17 separate written privacy policies into a single, easy-to-understand privacy policy last summer. He denied that AT&T sells information it collects, as did officials from Microsoft and Yahoo. Comcast said it shares information on user locations with third-party ad delivery firms, but declined to specify how much they earned from the sales of such data.

Markey said the responses would help inform lawmakers when they consider comprehensive privacy legislation, which they are expected to take up early next year. In addition to safeguarding children, he said, he's especially concerned about strong health privacy protections online.