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2007 was supposed to be the start of an advertising-industry downturn. But growth in agency revenues slowed from 8.8 percent in 2006 to 8.6 percent growth in 2007, according to AdAge's annual survey, and it's large part due to ever-increasing digital spends. The world's four largest agencies — Omnicom Group, WPP Group, Interpublic Group and Publicis Groupe — earned 12.3 percent of their revenue online in 2007. Digital accounted for 10.2 percent of the revenues for all U.S. agencies taking part in the survey.