Tech’s Titans Tiptoe Toward Monopoly

Imagine a not-too-distant future in which trustbusters force Facebook to sell off Instagram and WhatsApp. Imagine a time when Amazon’s cloud and delivery services are so dominant the company is broken up like AT&T. Imagine Google’s search or YouTube becoming regulated monopolies, like electricity and water.

Facebook Inc., Google parent Alphabet Inc. and Amazon.com Inc. are enjoying profit margins, market dominance and clout that, according to economists and historians, suggest they’re developing into a new category of...