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 percentage points, to 31.3 percent, comScore said in a report Thursday. Google’s nearest rival, Fox Interactive Media, which includes MySpace, accounted for just 4.4 percent of the market.

Interestingly, the top 10 video sites accounted for less than half of all videos viewed online. That means people are watching video here, there and everywhere on the Web. And they’re doing so more every day.

During November, 138 million people, or about three-quarters of Internet users in the United States, watched on average 3 hours and 15 minutes of online video, or 45 minutes more than they watched in January. That’s still less than the amount of time average Americans spend in front of their TVs each day. But the data are likely to be skewed by the smaller number of people who watch online video “for absurd amounts of time,” said Josh Bernoff, an analyst with Forrester Research.

“If you are CBS, the fact that people are watching a few hours of online video a month is of some concern,” Mr. Bernoff said. “But if there is some member of your audience who is there for one or two hours a day, you’ve lost them. They are never coming back.”

Those that have been lost by TV are likely to be found on YouTube, where 74 million people watched 39 videos each on average in November, according to comScore.