In the first quarter, the company had reported that it added 9 million more monthly active users than expected.
"What matters is MAU. Overall MAU were flat, and U.S. MAU was down," said Michael Pachter, managing director at Wedbush Securities.
Monthly active users, a key performance indicator for social networking services, is typically calculated by tabulating the number of users who have logged in and logged out during the 30-day period.
Twitter's second-quarter net loss widened as it took a $55 million impairment charge and revenue fell 4.7 percent.
The company has been trying to boost revenue through livestreaming deals, but suffered a setback in April when it lost a deal to livestream NFL games this year to Amazon.com Inc (AMZN.O).
Advertising revenue fell 8 percent to $489 million, but well exceeded the $458.1 million estimate.
The company's net loss widened to $116.5 million, or 16 cents per share, in the second quarter ended June 30, from $107.2 million, or 15 cents per share, a year earlier.
Excluding items, the company earned 8 cents per share.
Revenue fell about 4.7 percent to $573.9 million, the second time it has fallen since Twitter's debut in 2013.
Shares of Twitter fell 8.4 percent to $17.96 in premarket trading on Thursday, following a gain of 18 percent in the past three months.
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