‘A monopoly on the means of communication,” Robert Shea and Robert Anton Wilson wrote in “Leviathan,” their 1975 novel, “may define a ruling elite more precisely than the celebrated Marxian formula of ‘monopoly in the means of production.’ ” Bear that in mind when you hear this next statistic: In 2017 Google and Facebook have accounted for 84% of all digital advertising outside China, including 96% of its growth, according to an industry forecast this month from Zenith, Magna and GroupM.

Those figures should create more than the typical economic concerns about market concentration. Specifically, the tech duopoly’s dominance threatens the marketplace of ideas. Beyond advertising, Google and Facebook control how millions of people find their news. Americans are far likelier, collectively, to encounter articles via search engines and social media than on a news site’s home page.

Google is used for nearly 90% of online searches in the U.S. A Pew survey this summer found that the four most popular social-media sites for getting news are Facebook, YouTube (owned by Google), Twitter (which has a Google partnership), and Instagram (owned by Facebook). No more than 5% of Americans use another social-media platform to get news.

In a November speech, Ajit Pai, chairman of the Federal Communications Commission, argued that “edge providers” like social-media websites and search engines “routinely block or discriminate against content they don’t like.” Mr. Pai cited YouTube’s decision to place age restrictions on and pull ads from videos by conservative commentator Dennis Prager’s Prager University, including a video by Alan Dershowitz on Israel’s founding.

He also pointed to Twitter’s suspension of a pro-life campaign ad from Rep. Marsha Blackburn, an action that would have been illegal if done by a TV or radio station. Twitter has refused sponsored tweets from immigration opponents, saying its hate-speech policy is triggered by messages such as “the fiscal cost created by illegal immigrants of $746.3b compares to total a cost of deportation of $124.1b.”