In just 10 years, the world's five largest companies by market capitalization have all changed, save for one: Microsoft. Exxon Mobil, General Electric, Citigroup and Shell Oil are out and Apple, Alphabet (the parent company of Google), Amazon and Facebook have taken their place.

They're all tech companies, and each dominates its corner of the industry: Google has an 88 percent market share in search advertising, Facebook (and its subsidiaries Instagram, WhatsApp and Messenger) owns 77 percent of mobile social traffic and Amazon has a 74 percent share in the e-book market. In classic economic terms, all three are monopolies.

We have been transported back to the early 20th century, when arguments about "the curse of bigness" were advanced by President Woodrow Wilson's counselor, Louis Brandeis, before Wilson appointed him to the Supreme Court. Brandeis wanted to eliminate monopolies, because (in the words of his biographer Melvin Urofsky) "in a democratic society the existence of large centers of private power is dangerous to the continuing vitality of a free people." We need look no further than the conduct of the largest banks in the 2008 financial crisis or the role that Facebook and Google play in the "fake news" business to know that Brandeis was right.

While Brandeis generally opposed regulation — which, he worried, inevitably led to the corruption of the regulator — and instead advocated breaking up "bigness," he made an exception for "natural" monopolies, like telephone, water and power companies and railroads, where it made sense to have one or a few companies in control of an industry.