LONDON — Germany’s top antitrust enforcer, Andreas Mundt, recently asked a room full of lawyers, academics and regulators to imagine a wall filled with their personal information collected by Facebook and Google. He told them to picture it stocked with their data broken up into categories like finances, location, relationships and hobbies.

“That is you,” Mr. Mundt said. “And I promise you this wall knows you better than your wife.”

Few listened to Mr. Mundt when, a few years ago, he began raising alarms about the data collected by the tech giants. While online services like Facebook and Google did not charge a dime, people paid a high price by giving the companies so many personal details, he argued. And the people had no choice, because the companies had no real competitors.

But in February, his agency ruled that Facebook had broken the country’s law, and demanded that the company stop automatically sharing data among the services it owns, like Instagram and WhatsApp, or websites that use its “like” and “share” buttons. It was the first such ruling in Europe, putting in practice ideas that had never fully escaped academic and think-tank debates.

As American regulators and lawmakers intensify their scrutiny of Big Tech, there is a lot of discussion about whether or how they could accuse the companies of violating antitrust law. Often, regulators look to whether a company is causing consumer harm — a standard that can be hard to prove when a service is free.