The stock was down 11.5 percent at USD 34.30 in late morning trade, the lowest level since the Twitter initial public offering last November.
Under US securities lockup rules, some insiders were barred from selling for a period of time after the social network's IPO.
The lockup expired today, but Twitter co-founders Jack Dorsey and Evan Williams and chief executive Dick Costolo indicated they have no short-term plan to sell their shares.
Twitter made a spectacular Wall Street debut in November, rising from its USD 26 offering price to more than USD 70, but has been hurt by concerns about slowing growth and doubts on profitability.
Paul Ausick at the stock market website "24/7 Wall Street" said Twitter has also been hurt by a disappointing quarterly report which showed only modest user growth.
"One bit of good news is that CEO Dick Costolo told a channel that the company has no current plans for a secondary offering because most of the big shareholders do not need the cash immediately," Ausick said in a blog post.
"Twitter stock may be a safe buy for a while, but unless the company can crank up its subscriber numbers, all those shares lurking in the background are an albatross that the company does not need."
Twitter said in a regulatory filing in March that "the market price of our common stock could decline as a result of sales of a large number of shares of our common stock in the market, and the perception that these sales could occur may also depress the market price of our common stock."