In 2015, Barack Obama submitted to interviews with three YouTube stars, one of whom was notable for eating cereal out of a bathtub. It was a moment that opened a window into the media landscape of the future, after the mainstream media as we have known it — while also making that future seem basically absurd. A year later Donald Trump won the White House, and there was a rush to find the sources of his victory in the darker reaches of the internet, in misinformation factories and troll farms. It was another window into the media future — but...