Twitter's stock plunged 20.5 percent after the company said monthly users decreased in the second quarter.
The social media company also predicted further declines in the next few months.
It was the second-biggest loss for Twitter's stock since the company went public in 2013.
In percentage terms, the decline was slightly worse than Facebook's plunge the day before, but Facebook is a far more valuable company.
Twitter and Facebook are trying to reduce the amount of abuse and hate speech on their platforms, but that's affecting their growth, one of the things investors value the most.
Twitter has doubled in value over the past year as it became profitable for the first time and investors applauded its live video efforts.
Copyright AP - Associated Press