The trend reflects a change in the way consumers are navigating the Web. More online searches now take place on YouTube, the popular video site owned by Google, than on Yahoo, the No. 2 Web-search property. The change has companies including Pizza Hut, Universal Pictures and rethinking their search marketing strategies.

Even as No. 1 search engine Google's share of the online ad market -- including search ads -- continues to grow, marketers have started shifting ad purchases to other digital media, from social-networking sites to mobile phones. "Search is being redefined in a lot of different ways," says Peter Hershberg, managing partner at Reprise Media, a search marketing agency owned by Interpublic Group.

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