Pandora mobile usage rocketing. Mobile ads revenue less so.

CEO admits that 'we are ahead on where aggregate demand is for mobile advertising'.

US streaming music service Pandora is growing faster on mobile in terms of usage than in terms of mobile advertising revenues, says CEO Joe Kennedy.

"The sheer level of aggregate advertiser demand for mobile is limited," he tells Bloomberg. "Pandora is one of the top five players in mobile, so we generate a lot of inventory and are ahead on where aggregate demand is for mobile advertising."

60% of Pandora listening now happens on mobile devices, the company revealed in its recent IPO documents. Its smartphone and tablet apps have been the key ingredient in Pandora's growth in the last two years.

Analysts spoken to by Bloomberg express mixed opinions on Pandora's chances of solving its mobile challenge, with some suggesting that the growth in mobile advertising will eventually make all that usage worthwhile.

However, BTIG analyst Rich Greenfield suggests that for now, Pandora doesn't have the right kind of mobile advertising. "They don’t have a sales force to sell local ads the way your local radio station does. We think it's one of the biggest problems of the story."