‘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...