More content begets more viewers, which, in turn, beget more content. That’s the virtuous cycle — the network effect — that has propelled Google-owned YouTube to the No. 1 spot in online video.

Those network effects are helping YouTube outperform the growth of the overall market for online video, which itself is growing at a rapid clip. In September, Google sites accounted for 28.3 percent of all videos watched online, according to comScore. By November, Google’s share had grown by another 3 perc...