The stock had never closed below $30 per share, but it ended the day down more than 5.5 percent at $29.27. It bottomed below $29 in intraday trading Monday, down more than 6 percent.
Twitter has plummeted more than 11 percent since July 29, a day after executives warned that sustained user growth would come slowly. The weakness may not subside any time soon, said Bob Peck, an analyst at SunTrust Robinson Humphrey, on Monday.
"We would look for the fundamentals to change before we got more positive," he said in a CNBC "Squawk Alley" interview.
Last week, Twitter posted second-quarter earnings and revenue that topped expectations, but monthly active user growth that fell below estimates. Shares initially rose on the report, but quickly reversed after Chief Financial Officer Anthony Noto warned that "meaningful" user growth would not come for a "considerable" time.
"Our Q2 results show good progress in monetization, but we are not satisfied with our growth in audience," interim CEO Jack Dorsey said in a statement at the time.
On Monday, Twitter was about 35 percent lower than its first close in Nov. 2013, but remained above its IPO price of $26 per share.