User numbers fell to 335 million in the second quarter of the year, down one million from the previous three months.
During the quarter Twitter deleted many fake accounts - a move that chief executive Jack Dorsey said was reflected in the results.
The slide to just under $35 a share came despite Twitter reporting record quarterly profits.
It posted a profit of $100m, marking Twitter's third consecutive quarter in profits, with sales up 24% to a better-than-expected $711m.
Under pressure"Our second-quarter results reflect the work we're doing to ensure more people get value from Twitter every day," Mr Dorsey said.
"We want people to feel safe freely expressing themselves and have launched new tools to address problem behaviours that distort and distract from the public conversation."
Tech firms have been under pressure in several countries to be stricter over abusive content and misinformation, or "fake news", as well as political influence and protection of personal data.
