Google's YouTube continues as the preferred online video site of U.S. users, with a popularity that dwarfs its closest competitors, according to Nielsen Online.

During January, YouTube had almost 66.2 million unique viewers and streamed almost 2.6 billion clips, Nielsen Online said Wednesday.

Fox Interactive Media, the News Corp. unit that houses MySpace, came in second in streamed clips with 376.8 million and third in unique viewers with almost 19 million.

Yahoo ranked third in streams with a little over 299 million and second in unique viewers with 22.1 million.

Google's other sites, including Google Video, had 79.4 million streams and almost 13 million unique viewers, according to Nielsen Online.

As people embrace online video, it's expected that the medium will become a major vehicle for advertising, both for video ads and for ads within videos. All major Internet players are experimenting with video ad formats and strategies, trying to determine the most effective methods for generating ad revenue and promoting products and services via online video.

This week, Brian Cusack, YouTube sales team manager, told attendees at the eRetailer Summit in Miami that YouTube will soon give marketers more data about viewership of its videos.

By making more detailed metrics available, YouTube wants to give marketers a better understanding of the reach of their clips, and of their effectiveness at boosting brand awareness and sales.

He also said YouTube has only recently begun to be used as a marketing vehicle. YouTube only introduced significant advertising opportunities in last year's third quarter and is still very much learning how to make itself useful for these purposes, he said.

Other Internet companies, advertising agencies and corporate marketing departments are also striving to understand what are the best ways to use online video for advertising and marketing.

In the meantime, usage stats like the ones issued by Nielsen Online on Wednesday leave one thing clear: YouTube, at least for now, is the undisputed leader in online video.