Twitter Inc (TWTR) Shares Tumble Below IPO Price As Concerns Grow Over User Growth

Shares of Twitter Inc. (NYSE:TWTR) tumbled nearly 6 percent in afternoon trading Thursday, falling below $26 -- the company's initial public offering price. The selloff came after the company announced an overhaul of its mobile advertising platform with several new ad formats, renaming it "Twitter Audience Platform," 15 months after launching the "Twitter Publisher Network."