More than half of all initial public offerings in the U.S. this year are now trading below their offer price.

According to data tracker Dealogic, 63% of IPO listings in the country are “underwater”. Internet radio firm Pandora Media Inc. (NYSE:P), physicians’ hand-held software maker Epocrates Inc. (NASDAQ:EPOC), and Chinese social networking site Renren Inc. (NYSE:RENN) are all trading well below their IPO prices. Pandora is down over 40% since it debuted in June, Epocrates has declined 54% since February, and Renren has declined a whopping 62% since May.

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However, this year to date, IPOs have been performing better than stocks in general. IPOs were down 6.5% on average in 2011 as of close Tuesday, while the S&P 500 index was down 11.45%. Exactly half of 2011 IPOs are trading below the respective S&P 500 performance, which means the other half are trading at or above their initial offering prices.

Chinese Internet security software company Qihoo 360 Technology Co. Ltd. (NYSE:QIHU) closed Tuesday at $21.94, well above its $14.50 IPO price. Professional networking site LinkedIn (NYSE:LNKD) closed yesterday at $88.09 a share, nearly double its $45 IPO price. Real estate site Zillow (NASDAQ:Z) and Latin American McDonald’s (NYSE:MCD) franchiser Arcos Dorados Holdings Inc. (ARCO) both closed Tuesday above their offer prices and their highs on their first day of trading.

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According to Dealogic, professional services IPOs are the best performers, driven by Nielsen Holdings N.V. (NYSE:NLSN), which is up 20.87% from its offer price, and Zillow, up 86.38% from its offer price.