Compared with the first quarter last year, Twitter more than doubled its revenue, but the net loss widened by 390%. CEO Dick Costolo attributes the company’s revenue growth to increases in its user base and user engagement. The social network said it had 255 million monthly active users in the quarter ended March 31–a 6% rise from the 241 million users reported in the fourth quarter and a 25% increase in its base from the year-earlier period. Twitter was also able to boost engagement with improved recommendation tools, the rollout of a new mobile sign up process, and design tweaks to make the platform more approachable to mainstream users.

Twitter now claims 198 million mobile users, an 8% increase compared with the prior quarter. In total, mobile users make up 78% of Twitter’s base. Likewise, mobile ads comprise 80% of total advertising revenue. MoPub, the mobile ad exchange Twitter acquired in September for $350 million, “now reaches over 1 billion Android and iOS users, making it one of the–if not the–largest in-app mobile ad exchanges, and the only one at scale to offer publishers native in-app advertising,” Costolo said in a conference call with investors. “To be very clear, there are few other companies with this reach.”

In line with Twitter’s user growth, the number of timeline views rose 6% to 157 billion from the fourth quarter. After a strong holiday season, advertising revenue continued to grow to $226 million from $220 million last quarter. Twitter made $1.44 per thousand timeline views, compared with $1.49 in the previous period. Other sources of revenue include data licensing at $24 million and international revenue at $70 million.

“These metrics also benefited from new device activations and significant live events in Q1,” chief financial officer Mike Gupta said, warning investors of fewer live events in the second quarter. Live events present an opportunity to re-engage users, Costolo noted. Pointing to the 3.3 billion tweet impressions around the Oscars, he said the event is “a tiny example of the ubiquity, the almost omnipresence, we have in the world to be in front of users.”

Twitter has raised its revenue guidance for the second quarter to $280 million, up from $270 million. But expenses for the coming quarter will increase due to acquisitions, including social data provider Gnip, and other investments. The company also projects revenue to hit $1.2 to $1.25 billion in 2014. In the year to date, Twitter shares have dropped close to 40% in value.