The source was not authorised to speak publicly and declined to be named. Groupon was not immediately available for comment. Pandora declined to comment. LinkedIn plans to go public this year, sources have said. Microblogging site Twitter and Zynga, which develops social online games, also are seen as potential IPO candidates. Groupon has aggressively been raising capital. In December it sold a US$500 million equity stake, and on Monday, it announced it had completed a US$950 million round of financing. (Reuters)