But the internet services from Google and Facebook are free to consumers. The old yardstick no longer applies, prompting calls for new rules for the digital giants.

Prices matter, Mr. Delrahim said, but he emphasized that they were only one consideration in antitrust enforcement. Others, he said, are the impact a dominant company’s actions have on consumer choice, quality of service and innovation.

Free offerings are not new in antitrust law.

In the Microsoft case in the 1990s, Mr. Delrahim noted, the company bundled its Internet Explorer browsing software to its dominant Windows system as a free add-on. That undermined competition from a new competitor, Netscape Communications, the commercial pioneer in internet browsing software.

At the time, Mr. Delrahim was a young lawyer working on the Senate Judiciary Committee, which held hearings on Microsoft. Asked if he were in the Justice Department then, would he have pursued the Microsoft suit?

“I absolutely think that was a case to bring,” Mr. Delrahim replied.

Not surprisingly, Mr. Gates disagreed. And he recalled his time at the center of an antitrust case and the personal and corporate energy consumed by it.

Microsoft was found guilty of repeatedly violating antitrust laws, and in a settlement agreed to stop some tactics.