Twitter had a strong public stock debut Thursday in the most highly anticipated initial public offering since Facebook's last year. But one risk investors face is Twitter burnout. Twitter is trading under the ticker symbol "TWTR." Twitter's stock opened at $45.10, or 73 percent above its $26 IPO price. The opening price values Twitter at more than $31 billion based on its outstanding stock, options and restricted stock that'll be available after the IPO. Expect some changes throughout the day, though: It has traded as high as $50.09 and is now at $45.97.