The shares of popular but unprofitable Internet radio service Pandora Media Inc. soared more than 50 percent in its market debut Wednesday.

Its shares opened at $20 a share and rose as high as $26 in early trading on Wednesday, up from its offering price of $16. At its high, that valued Pandora at $4.2 billion.

That is nearly twice the current value of AOL Inc. but a fraction of such Internet behemoths as Google Inc. or even Yahoo Inc.

Pandora's offering comes amid a fervor for high-profile Internet IPOs that have just begun to trickle in. The professional networking service LinkedIn Corp. was among the first to its public debut among the latest generation of Web startups.

Daily deals site Groupon Inc. has also filed to go public, though it has not yet provided details on its expected price range or amount of shares it plans to sell. "FarmVille" maker Zynga Inc. is also expected to make its public debut soon.

Facebook, the owner of the world's largest social network, has indicated it will file its IPO documents before May 2012. Based on a Goldman Sachs Group investment, Facebook was valued at $50 billion at the beginning of this year.

Pandora, based in Oakland, Calif., got its start in 2000 as a music recommendation service, then known as Savage Beast Technologies. It changed its name in 2005 when it launched an Internet radio service that lets people stream music over the Web, letting users create custom stations based on songs, genres or artists.

Pandora has 94 million registered users and it makes most of its money from advertising. In the most recent fiscal year, Pandora booked a loss of $1.8 million before paying dividends on preferred stock. Its revenue was $138 million.

Shares can be volatile on their first day trading. LinkedIn issued its stock $45 in mid-May. The shares soared above $100 before noon on the day they hit the market and closed at $94.25 on a trading volume of 30 million shares.

On Wednesday, LinkedIn shares fell 2.3 percent to $74.62 in morning trading.