American technology firms are especially worried because they routinely transfer so much information across the Atlantic. “International data transfers are the lifeblood of the digital economy,” said Townsend Feehan, chief executive of IAB Europe, which represents online advertising companies including Google as well as small start-ups. The ruling “brings with it significant uncertainty as to the future possibility for such transfers.”

As Mr. Schrems sees it, however, what is at stake is a deeper conflict between the European legal view of privacy as a right equivalent to free speech and that of the United States, where consumers are asked to read and agree to a company’s terms of service and decide what’s best for themselves. “We only do this in the privacy field — dump all the responsibility on the user,” Mr. Schrems said. He pointed out that consumers are not expected to make decisions about other complex issues, like food or building safety. “In a civilized society,” he said, “you expect that if you walk into a building it’s not going to collapse on your head.”

Beyond Big Brother

“I’m not a big privacy person,” Mr. Schrems told me in May over brunch at the Naschmarkt, a hip outdoor market in Vienna. That’s fortunate, since his activism has had the paradoxical effect of making him a public figure in Germany and Austria — he’s even appeared on the cover of the German tabloid Bild-Zeitung. Analytical by nature, Mr. Schrems is more interested in privacy in principle than in practice; he says he’s not hiding anything but that he wants to be able to decide what he shares with whom. He doesn’t tell most journalists he is gay, information he volunteered in an interview, for example, “because then people think you only want to keep things private because you’re gay.”

However much people want to hold on to their privacy, they nonetheless inadvertently reveal all sorts of things when they go online, including habits, sexual orientation and political beliefs. Data gathered online is sometimes sold, shared or combined with information from mobile phones or offline sources. All of this information is a vital raw material for a digital advertising business expected to be worth more than $80 billion worldwide by 2018.

“Surveillance,” wrote Bruce Schneier, a leading computer security analyst, “is the business model of the Internet.” Big Brother is no longer the only threat to privacy, and Europe has struggled to regulate the gossipy circle of consumer-data-collecting companies. Facebook currently faces challenges from five European regulators, including a Dutch-led investigation into how the company uses data from services like Instagram and WhatsApp and a Belgian effort to stop it from tracking consumers who have not joined the service.

Mr. Schrems, lawmakers and various regulators are essentially asking why consumers don’t have more control over the information gathered by their computers and phones — and perhaps soon by their smartwatches and self-driving cars. “This is something we see as a fundamental value,” Mr. Schrems said. To drive home the analogy for an American, he put it this way: “You can’t say, ‘Oh, I don’t feel like applying the First Amendment.’ ”