EMarketer: Mobile video ad spending is growing faster than any other digital advertising format in the U.S., but it’s still straining to catch up with desktop video — let alone TV — according to a new eMarketer report. In the U.S., spending on mobile video advertising will grow more than 70% to reach $2.62 billion in 2015 — over one-third of the estimated $7.77 billion to be spent on digital video ads. But it still underperforms as a share of digital video spending in comparison with desktop. “Challenges with the format are causing growing pains for the industry.”