Some libertarians argue that social media companies are private and therefore can suppress any speech they please. In normal free market environments, in which competitors could challenge them, this would be true and simple enough.

But Supreme Court Justice Clarence Thomas this week explained the problem with this line of thinking as it pertains to Twitter, Facebook, Google, and Amazon. “A person always could choose to avoid the toll bridge or train and instead swim the Charles River or hike the Oregon Trail,” Thomas wrote. “But in assessing whether a company exercises substantial market power, what matters is whether the alternatives are comparable.”