Twitter's share price has fallen by nearly a quarter after the micro-blogging site reported slower growth in user numbers.
The shares had fallen 24 percent to $50.01 as trading neared its end in New York on Thursday.
The US company also reported a $511m quarterly loss, signalling fears that the short online messaging service was losing popularity.
Twitter had reported stronger-than-expected revenue and adjusted earnings in its fourth-quarter financial report on Wednesday, its first as a publicly traded company.
The site had 241 million users at the end of 2013, adding just nine million new monthly users since it went public in November, suggesting only modest growth at a time when investors were looking for a surge.
It was adding an average of 16 million new accounts in each of the first three quarters of the year.
Stifel analyst Jordan Rohan downgraded Twitter, noting that "user growth metrics faltered" and adding that "the company did a poor job explaining how and why the lower user growth was a temporary phenomenon," the AFP news agency reported.
San Francisco-based Twitter's shares have soared since its November initial public offering, causing many analysts to call it too expensive.
Dick Costolo, the company's chief executive, sought to reassure analysts and investors during Wednesday's conference call, saying that changes Twitter made to the short messaging service during the fourth quarter are starting to pay off.
Sterne Agee, an analyst Arvind Bhatia, said the company's share price was expensive compared to similar companies. He set a target price of $43 for Twitter's stock, which is still more expensive than Facebook and LinkedIn, based on expected revenue for the companies, the analyst wrote in a note to investors.
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