Internet radio service Pandora passed 50 million registered users late last week, co-founder Tim Westergren announced at the company blog. "The growth of mobile has clearly helped a ton,” wrote Westergren, adding that an increasing number of new users are coming from smartphones, connected home devices and Blu-ray players.

The service had 48 million registered users at the end of February, which means it grew by two million users in 25 days – or 80,000 new registered users per day. That’s a big uptick from the 60,000 per day Pandora was adding when Westergren talked to Billboard in May 2009 but short of the 85,000 new daily registrants Westergren claimed in a recent New York City town hall meet up.

Pandora, a privately held company, says Q4 2009 was its first ever profitable quarter. It finished 2009 with revenue of $50 million. Since taking $1.5 million in angel investments in 1999, the company has received investments from Crosslink Capital, Labrador Venture Partners, Selby Venture Partners, Greylock Partners and WaldenVC. Greylock has funded such hot properties as Facebook, Digg and LinkedIn. WaldenVC put money into Snocap (acquired by Imeem in 2008) while Labrador has invested in interactive video company Ustream.