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.”

The stock dropped to an all-time low of $25.92 in afternoon trading, with shares closing the session down 5.8 percent to $26. 

Twitter shares have extended losses by nearly 17 percent in the last three weeks since posting second-quarter results on July 29. The social media company warned investors would not see "sustained, meaningful" user growth for a "considerable period of time," overshadowing better than expected earnings and solid full-year revenue guidance.

The recent losses also come after Twitter announced in June that CEO Dick Costolo would step down on July 1 after facing pressure as expectations mounted for the social networking giant to generate high user growth. Jack Dorsey, co-founder of Twitter, is now the interim chief executive officer.

There was at least one bright spot in the company's earnings. Twitter’s revenue soared 61 percent from a year ago, topping Wall Street forecasts, driven by growth in advertising revenue. Although Dorsey was pleased with the revenue results, he was disappointed with the slow user growth.“This is unacceptable and we’re not happy about it,” Dorsey told shareholders during a conference call last month. 

Shares of Twitter have lost nearly 38 percent of their value since the close of its first trading day on Nov. 7, 2013. Even before the IPO, Costolo faced pressure as expectations mounted for the social networking giant to generate high user growth. The stock soared more than 70 percent on the first day of trading to close at $44.90 a share, up from the initial pricing of $26 set ahead of its debut.

Since then, the company has faced tough shareholder questions about slowing user growth. Twitter reported 304 million active monthly users for the second quarter of 2015, up 12 percent from a year earlier. But the gains pale in comparison to Facebook Inc., which has more active monthly users than Twitter and LinkedIn combined. Facebook announced in April that it officially has more monthly active users (1.44 billion) than China has people (1.4 billion).

Facebook’s active monthly users rose to 1.49 billion in the second quarter, an increase of 13 from a year ago, as of June 30.

S&P Capital IQ lowered its 12-month price target on Twitter last month by $6 to $44, and reiterated its Buy rating on the company.