Some fund managers say that Twitter Inc.'s $12.8 billion valuation is set to get much higher, Bloomberg reported. The $12.8 billion figure is derived from the fair value that Twitter put on its shares in an initial public offering filing last week, but Ironfire Capital LLC and Gamco Investors Inc. project that the company will be worth $15 billion to $20 billion once it begins trading. The firms expect that Twitter can increase revenue at a quick pace as it expands globally and obtains advertising from companies. The social media site has more than doubled revenue annually as companies advertise on the service, but it still isn't turning a profit.