Shares of Pandora Media Inc. (NYSE:P) plunged $1.05, or nearly 6% to $16.88 on Thursday after the company's stock drew back on the same day that Spotify was released in the U.S.Earlier this week, Pandora announced they reached 100 million registered users and 36 million monthly active users of its music-streaming service. Since the company went public in June, shares of Pandora have varied.