From the 1930s through the 1980s, antitrust lawyers worried a lot about vertical integration: companies that owned every step of production, from mine to showroom. In 1948, the Department of Justice won a case forcing the major movie producers to sell their chains of movie theaters.

The Supreme Court’s decision in U.S. v. Paramount Pictures incidentally destabilized the careers of many B-movie actors who had until then enjoyed steady, predictable salaries under the old studio system. The shock jolted one of those actors, Ronald Reagan, from his former New Deal liberalism to burning rage against an over-intrusive federal government.

Indeed, the old concern with vertical integration did become progressively more intrusive. In the 1960s, makers of car radios brought a spate of lawsuits to stop carmakers from selling cars with radios preinstalled. Could cameras be sold with film? Insulin together with the infusion apparatus? How could an economy innovate if every product upgrade required government approval? Just think of how much of the office equipment of the 1980s—calendar, camera, calculator, dictionary—comes bundled in a modern phone!

When Reagan reached the presidency, in 1981, he oversaw the reinvention of antitrust law. The new thinking on antitrust—most powerfully expressed in Robert Bork’s 1978 book, The Antitrust Paradox—denounced the old concern with vertical integration. The Justice Department, Bork argued, should zealously police mergers between companies that competed directly against one another: horizontal competition, in the argot. What happened up and down the product chain should be left to Mr. Market to decide. The Comcast–NBC deal was approved by that logic.

Unnervingly, however, the uses of market power that we confront in the 2010s look a lot more like the old motion-picture studios trying to control every inch of film content in their theaters than like car-radio makers trying to force consumers to buy two products at a higher price instead of one convenient bundle of car and radio at a lower price. Facebook seeks to seize for itself all of the value created by its users. Companies such as Comcast hope to use control of the content consumers want to extract purchases of content that consumers want less. Time Warner, the DOJ fears, hopes to extend this grasp to the emerging mobile world: maintaining the high profits of the old cable industry even as Americans sever cable’s physical cords.

As social media emerge as the nation’s, and the world’s, true public square, hard questions arise about their owners claims to be mere platforms. Does Facebook really have no duty to police advertisers who request, “No blacks, please?” Can Twitter stand aside as its platform is used for harassment and threats?

The deregulated, postindustrial world of the 1980s and ’90s seemed to banish old fears of industrial concentration. The world of Google, Facebook, and Amazon looks a lot more like the world dominated by U.S. Steel and General Motors. Suddenly, formerly antique antitrust ideas again seem relevant to our time.