Average monthly active users (MAU) for the first quarter totaled 330 million, down 1.8% from 336 million in the first quarter of 2018.
"We are taking a more proactive approach to reducing abuse and its effects on Twitter", Dorsey said Tuesday in a statement.
Revenue was boosted by ad sales that also jumped 18% to $679m.
The firm was trying to reduce the burden on victims by using artificial intelligence to spot abusive tweets and to take them down before they were reported, he said. That compares with 126 million in the fourth quarter.
Twitter shifted focus to daily users for the first time in the December quarter, arguing that it's a more meaningful metric than monthly visitors because the company aims to drive people to the service every day.
Twitter has posted better-than-expected first-quarter revenue and a surprise rise in the number of monthly active users, a sign that the social media platform may be on a sustainable path after a year of stagnant user growth. It has been removing thousands of spam and suspicious accounts.
Analysts were encouraged by signs Twitter had turned a corner in monthly user growth, but said that the new way of measuring users could make comparisons between Twitter and rivals like Facebook Inc more hard.
"The best thing ever to happen to Twitter is Donald Trump", Trump tweeted on Tuesday, after Twitter reported its results, attributing the comment to Fox Business Network host Maria Bartiromo. "Investors are very cognizant of the investments we've seen at Facebook to try and get ahead of its regulatory issues, and I think people are not quite sure what that means for Twitter".
While Twitter was able to attract more people to its site, one prolific tweeter found reason to grouse.
She said Twitter's revenue beat expectations, with most of the growth coming from the U.S., even though most new users were worldwide. The company is also rolling out a Snapchat-like camera feature that lets users post videos or photos in a swipe.
Advertisers, which provide Twitter with almost all of its revenue, have welcomed those moves, but the company is still grappling with the prospect of regulation in the United States and overseas and allegations that its clean-up amounts to censorship. Facebook has climbed more than 38 per cent in 2019, while Snap Inc. has more than doubled.
First-quarter adjusted earnings per share came in at 37 cents, jumping ahead of consensus estimates of 15 cents per share and the year-ago quarter's 16 cents per share.
On the finance side, the company saw its operating income rise from $75 million in the same period past year to $94 million now.
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