The core distinction, industry executives say, is that Web companies do not have the power to prosecute anyone, as law enforcement agencies do. Rather, the industry is trying to discover users’ tastes and preferences, said Michael Moritz, a venture capitalist and chairman of Sequoia Capital in Menlo Park, Calif. “If you go to the heart of these companies, you have people wanting to make sure their products and services are as useful as possible to consumers,” he said. “They know that’s the way they’ll be able to flourish. The last thing they want to do is raise the ire of consumers.”

But hang on, critics say. Commercial data collection has its own dangers. As is widely known by now, posts on Twitter or Facebook can affect college admissions or prospects for employment. And the very collection of data by the likes of Facebook, Microsoft and Verizon creates a vast reservoir of information that intelligence agencies can tap into.

In any event, the lines between the two sets of data, these critics say, are blurring. “I’m not saying evil corporations are doing this intentionally,” Mr. Acquisti said. “At the same time, their technologies have created this infrastructure, which makes universal surveillance possible.”