Costolo is expecting those percentages to grow, along with percentages of favorites and retweets, which grew 26 percent in the first quarter.
"The areas of focus for us have been increasing the value of the Timeline," Costolo said. "Our net new users are just engaged as our existing users. We see that impact flow through to monetization. The engagement rates we're seeing are fantastic."
Costolo is happy, but Wall Street isn't. Twitter's stock plunged to new lows after reporting its first-quarter numbers.
A bright spot: MoPub, a mobile ad network that lets brands buy ads on other apps that aren't related to Twitter, is now part of the Twitter fold and reaches 1 billion iOS and Android apps. Even if Twitter struggles to grow, MoPub will provide a robust ad exchange to contribute to its parent company's bottom line—unless Facebook launches its own ad network, as it's expected to Wednesday at its F8 developers conference.
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