Last February’s Super Bowl delivered 114 million U.S. viewers, making it the most-watched television broadcast in American history. In June, a YouTube user uploaded a video of toddlers playing in a tub of colored balls, then naming the colors. It has been watched 249 million times. In fairness, the football game went for hours, while The Ball Pit Show is about two minutes. But then, the game is over, while the video is still running—and it doesn’t even rank among the 25 most-watched YouTube clips this year.

At the top, with...