Twitter Inc's said average monthly active users have stalled in the fourth quarter - the first flat quarter sequentially since the company listed in 2013.
Twitter shares fell 6.6 percent at US$13.99 (NZ$20.94) in after-hours trading, after the social media company's revenue forecast for the current quarter missed analysts' expectations.
The weak results will stoke investor concerns over the company's stalling growth and the effectiveness of Chief Executive Jack Dorsey's turnaround strategy.
The company's shares have more than halved in value since Dorsey, one of the founders, returned to Twitter in July.
The microblogging service forecast first-quarter revenue of between US$595 million and US$610 million, well below the average analyst estimate of US$627.1 million according to Thomson Reuters I/B/E/S.
Twitter said in a filing it had 320 million average monthly active users in the quarter, unchanged from the third quarter and lagging a forecast for 323 million users from RBC Capital Markets.
The company, facing slowing user growth, has been experimenting under Dorsey, who became interim CEO in July and then CEO in October, to make its website more engaging.
Twitter made a dramatic product change earlier in the day, saying it will recast the way it displays tweets on its home page - customising them to individual users.
The change to the timeline on the home page is designed to appeal to advertisers by giving more prominence to tweets that advertisers pay for.
But some analysts have said that the earlier efforts including Moments, which showcases Twitter's best tweets and content, have not taken off.
An exodus of some top executives last month added to investor concerns about the company's ability to reignite stalled growth.
Revenue rose 48.3 percent to US$710.5 million in the quarter ended December 31.
Twitter's net loss shrank to US$90.2 million, or 13 cents per share, in the fourth quarter ended December 31 from US$125.4 million, or 20 cents per share, a year earlier. Excluding items, it earned 16 cents per share.
Analysts had expected a profit of 12 cents per share on revenue of US$709.9 million.