Twitter (TWTR) opened for trading at $45.10 and is currently at $45.98, up 76.9% from an IPO price of $26. The company has officially avoided a repeat of Facebook's messy IPO.
Based on common stock outstanding, Twitter currently has a valuation of $25.1B, nearly matching LinkedIn's $25.8B. If one accounts for options and restricted stock units, Twitter's market cap is a few billion higher than that.
Shares trade at ~40x estimated 2013 sales.