Pandora has 75m users

"First, a heartfelt thanks to all of you for continuing to be such engaged listeners and such wonderful evangelists. In 2010 we doubled our audience to over 75 million people! And that was mostly thanks to continued word of mouth. For that we are deeply appreciative.
As always, interacting with listeners continues to be our greatest joy, and greatest source of feedback. You now send us over 25,000 emails every month. What an incredible fountain of information, constructive feedback, and validation that continues to be... and those thumbs! In 2010 alone, you thumbed over 3 billion songs (mostly thumbs-up... phew!)."
Source:  Tim Westergren of Pandora, in the Pandora blog, 12th January 2011