More than 100 million unique viewers watched 5.9 billion videos on Google sites in December 2008, with YouTube accounting for almost 100 percent of the views, according to comScore Video Metrix, eMarketer reported Thursday.

However, marketers are questioning how much high levels of traffic will translate into revenue increases.

Typically, marketers want to see ads placed with content they are comfortable with, or want consumers to relate their brands to. This is not the case with YouTube, in which most content is user-generated, and not trusted by marketers, said David Hallerman, an eMarketer analyst.

Hallerman explained that most YouTube content is short-form and can support only ads 15 seconds long or shorter. Most advertisers are inclined to spend money for online video advertising when video shows are longer, according to eMarketer.

The popular short forms on the video site will continue to distract viewers, even though YouTube has struck deals for longer and more professional videos, Hallerman added.

Hallerman cited Hulu as an example for a simple video site that facilitates an easy and direct link between the audience, video and advertiser, eMarketer reported.