Selling ads requires that we demonstrate to advertisers that our service has substantial reach, and we rely on third parties to quantify the reach of our service. These third-party ratings may not reflect our true listening audience and the third parties may change their methodologies, either of which could adversely impact our business. Third-party independent rating agencies have not yet developed rating systems that comprehensively and accurately measure the reach of our service.
Interesting note from the risks section of Pandora’s IPO Prospectus

Notes