Google-owned YouTube is still the king of online video sites but when it comes to selling video ads, NBC's Hulu remains unbeatable, according to the latest comScore report.

According to the May report of Online analytics tracker comScore, YouTube helped Google pull in a whopping 147.2 million unique visitors. The number is daunting as the total number of US Internet users is 176.3 million, comScore said. In other words, nearly each and every Internet users in the US have visited YouTube at least once, making it the Internet’s top source for user-generated video content site.

VEVO ranked a distant second with 60.3 million unique visitors, followed by Yahoo! sites (55.5 million) and Facebook.com (48.2 million). Hulu is at the bottom with 28.5 million unique visitors.

However, Hulu is the undisputed leader when it comes to selling video ads.

According to comScore, Hulu sold 1.3 billion video ads, followed by Tremor Media Video Network (700.7 million) and Adap.tv (641.6 million). The total number of video ads played in May was 4.6 billion, which means at least one out of three video ads played in May was on Hulu.

BrightRoll Video Network (564.9 million) and Undertone (291.2 million) rounded up the top five.

comScore also threw up other interesting figures:

> Each visitor watched an average of 311.2 minutes (5.2 hours) of video in May
> 45 percent of the total US population sees video ads about 34 times each month
> 83.3 percent of the total US Internet audience watches videos online
> Online videos are on average 5.2 minutes long, with video ads running for about 24 seconds

The report is going to give YouTube something to think about as Hulu seems to be generating far more revenue than YouTube when it comes to video ads.

Not surprisingly, YouTube is currently in talks with professional basketball and hockey associations in the U.S. so that it can offer live streaming sporting events to its audience. YouTube is also seeking live video tie-ups with soccer leagues in Europe. In India, YouTube tied up Indian Premiere League to offer live streaming cricket matches. The deal turned out to be a money-spinner as it brought YouTube a share of the ad revenue for games and a total of 55 million views from over 250 countries.

Meanwhile, Hulu is preparing to file an IPO which could help it raise up to $300 million and give it an estimated market cap of $2 billion.

The General Electric’s NBC Universal, Walt Disney Co and News Corp-backed video site, which was launched in 2007 to bring TV shows online and provide better video quality than YouTube, plans to use the money to take on its main rival NetFlix and steal market share from other video content providers such as Google, Apple and Amazon.