Even companies that pledge not to share online consumer information with an outside party often store more data, and for longer periods of time, than most consumers realize, or would agree to if they knew, Hoofnagle said.

"From the consumer perspective, many, many thousands of companies track everything they do online and offline, maintain profiles of them and sell them to whoever will pay the most for it," said Steven Gal, CEO of ProQuo, a start-up that allows users to choose which paper junk mail to stop receiving from different sources. Such companies don't let consumers see their profiles, or interact with these profiles, resulting in a lot of junk mail and spam, he said.

Despite the extensive profiling now under way online, consumers still have more control than those in the offline world, Gal said. Large companies, in particular, have begun to disclose more to consumers and allow them to opt out of tracking. Many are beginning to exercise that option, he said.

"I think consumers know much more in the online world. But in general, I don't think most want to understand how it works," he said. "It's kind of scary. It makes them feel overwhelmed and like they have no control."

Concerns about online tracking and profiling have spilled into the open in recent days ahead of the FTC event to discuss the issue.

Recently a group of nine privacy organizations asked the FTC to consider a Do Not Track list to protect people from having their online activities unknowingly tracked and used by marketers. The group also wants the formal definition of the term "personally identifiable information" updated, and it said Internet advertisers should be forced to provide more robust disclosures on any behavioral tracking they are doing.