Facebook is now the third most popular video website in the United States with 46.6 million unique viewers per month, according to new data from comScore.

The social networking leaped one position to come in at number three for July 2010, behind Google, owner of YouTube, with 143.2 million unique visitors per month and Yahoo! sites with 55.1 million. Microsoft sites ranked closely behind Facebook with 45.6 million.

Hulu.com, which was ranked tenth in terms of monthly unique visitors, had high levels of engagement and exposure to ads. Hulu viewers watched an an average of 158 minutes per month each and was the top site for video advertising with 783 million ad impressions over the month.

By contrast, Facebook did not make the top ten for video advertising. However, the study only included streaming video advertising, not other forms such as overlays, branded players, matching banner ads or home-page ads. A total of 84.9 per cent of US internet users watched online video in July and 3.6 billion video ads were delivered across the industry. The average online content video was 4.8 minutes long and the average online video ad was 0.4 minutes. Video ads accounted for 9.8 per cent of all videos viewed and 0.9 per cent of time spent video online.

While Facebook can take pride in its growing popularity in the world of online video, it’s my belief that this actually could become a problem for them. Currently Facebook hosts the videos but doesn’t sell pre-roll or layover advertising. The pages the videos are on likely have advertising in the side panels but unless the site can find a way to make money from the videos directly, the cost of hosting all that multimedia on their servers could become prohibitive. The site has already been forced to double the size of its planned data center in Oregon because of the huge growth in user numbers over the past year.