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.