) --


(FB) - Get Report

fourth-quarter earnings beat

Wall Street's estimates

, helped by ad revenue on mobile devices such as smartphones.

In a phone interview with


, Chief Financial Officer David Ebersman said that "2012 was the year Facebook became a mobile company. Mobile provides a huge opportunity for Facebook, and we're working to make sure our ad products work well on mobile."

Mobile revenue represented 23% of advertising sales in the fourth quarter, up from 14% in the third quarter. Total advertising revenue was $1.585 billion.

While mobile continues to become a bigger part of the company, "it's still very early in mobile," Ebersman said. "It's still early in bringing products to market, and we want to make sure our ad products work well."

For the first time, the rapidly growing company experienced mobile daily active users (DAU) surpass web DAUs. "This is exactly what we expect," Ebersman, 42, said. "The rapid growth of MAU provides greater utility."

Costs and expenses rose sharply, jumping 82% to $1.06 billion, but the former


CFO isn't worried. "Our key issues are investments in our people

Facebook ended the quarter with over 4,600 employees and infrastructure," Ebersman said. "We are investing in engagement and monetization, and we're showing the results."

-- Written by Chris Ciaccia in New York

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