eMarketer's recent data shows the mobile ad revenue market growing from $2.61 billion this year to $6.61 billion by 2014 and nearly $12 billion by 2016. Google ( GOOG) leads and will continue to lead. Beyond that, the hierarchy will change over the next few years. There's a reason why Pandora ( P) will finish 2012 in second place. Not only does it have a large and established audience, Pandora anticipated the mobile transformation long before Facebook did. It would have been tough for Pandora not to get a head start. When Apple ( AAPL) introduced iPhone, Pandora's audience went through the roof. Its platform went from one that, by and large, was tied to a fixed location to one users could take with them just about anywhere.