You’ve read all of your free items this month. To register, get added benefits and unlimited access to articles, Become a Member. Already a Member? Sign in. Advertising Age Article Abstract Olympics, Mobile Drive Three New Ad Forecasts, but Europe Stays a Concern Mobile Now the Main Driver of Global Ad Spend Growth, ZenithOptimedia Says By Alexandra Bruell, Published on December 08, 2013 1 Want more Access, Content & Connections? Upgrade to Membership. We are glad you are enjoying Advertising Age. To get uninterrupted access and additional benefits, become a member today. Become a Member Already a member? Log in.