Facebook co-founder Mark Zuckerberg said, in a speech last week at Georgetown University, that his social media megacorp and its Big Tech peers “have decentralized power by putting it directly into people’s hands.”

That sounds comforting and egalitarian, but a lot of people worry these days that they’ve actually centralized power—around themselves. This has become ever more obvious since Russian agents used Facebook in an attempt to manipulate the U.S. public in the 2016 election. It’s clearer every time somebody searches...