eMarketer estimates U.S. digital ad spending will rise 14% to $41.9 billion in 2013 -- and that $7.7 billion of that total will go toward mobile ads. The research group also estimates marketers' desktop ad spending will peak at $35.39 billion next year, up from the projected $34.29 billion in 2013. By 2015, desktop spending will drop slightly to $35.26 billion, and will decrease further in 2016 and 2017, when it's expected to total $32.51 billion.