Of course, this never stopped YouTube, Twitter, Skype, or Groupon (which itself is in the midst of IPO mania right now). The difference is that YouTube, Twitter and Skype never went public -- and 2 of the 3 were gobbled up by two of the few handful of companies that could afford to lose big initially on the service itself for the sake of long-term strategic benefits that are not necessarily tied only to that service. Once Pandora goes public, that kind of "big fish" result is not in the cards (or, at a minimum, extremely difficult).
I love the service. But, not so much to invest in it. I am clicking Pandora's "thumbs down" on this one.