Online video outlets are finally starting to chip away at television's hold on advertisers.

Several major advertisers, including MasterCard, Mondelez International and Verizon Wireless in the past year have moved a portion of the money they previously spent on television to online outlets, conscious that viewers are more frequently watching video online, ad executives say. And with online outlets in recent weeks unveiling plans to ramp up their programming, more shifts are likely, say media buyers.

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