Raising new questions about the viability of online video as a revenue source, the sales manager of Google's YouTube has told the ClickZ conference on online video in New York that his sales team has been able to sell ads against less than 3 percent of the videos posted on the site. Brian Cusack also disclosed that the biggest advertisers are television networks, who presumably are buying spots to hype their television shows, and movie studios. Cusack also said that the YouTube sales staff has also begun "making progress" with other advertisers, including home improvement retailers, who buy ads against "how to" videos. The Silicon Valley Insider observed today that many of the biggest viral hits on the site, like last week's "Homemade Porn" video, run without advertising.