Here’s a direct quote (below) from TechCrunch (view the post):
Pandora has announced that it surpassed 40 million registered users earlier this month. That means the service had doubled its size in 2009. And it’s adding 600,000 new registered users a week now. Even more remarkable is that half of those new users are coming from mobile devices. And of those, the iPhone continues to lead the way with 10 million Pandora users of its own. That number has grown some 400% this year.