The growth of the mobile video audience is impressive. the number of smart phone video viewers in 2014 to hit nearly 87 million, or more than one-quarter of the total US population. Tablet usage is also rising rapidly, and a larger screen and lean-back usage make tablets much better for video consumption than smartphones. With US ad spending for mobile video set to quadruple from this year’s $518 million estimate to nearly $2.1 billion in 2016, there’s much at stake here.