Twitter is valued at approximately $11 billion

"A rough valuation for Twitter can be established through where its shares trade in online secondary markets, though a firm figure is difficult. “Using the secondary market for shares to mark enterprise value is a very difficult and opaque process. It is a rumor-rich and special-share-class soup,” says Greencrest’s Max Wolff. “That said, Twitter is up since the Facebook IPO and is now valued at northward of $11 billion. This makes sense as growth in users and new monetization efforts are both yielding fruit and pointing toward a good 2013 for Twitter.” A brief history of Twitter’s valuation: in July 2011, a funding round pegged it at $8 billion, up from a $3.7 billion figure that emerged seven months earlier. For perspective, Facebook currently has a market cap of $60.8 billion. Zynga ($1.9 billion) and Groupon ($3.4 billion) are much smaller."

Source:  Forbes, 3rd January 2013