Facebook's mobile ad revenue doubles in fourth quarter SAN FRANCISCO (Reuters) - Facebook Inc doubled its mobile advertising from the previous quarter as the world's No. 1 social network expanded its business aggressively onto handheld devices. But the growth trailed some of Wall Street's most aggressive estimates. Extending its reach on mobile devices is crucial as more and more of its 1 billion-plus users begin to access its network via smartphones and tablets. The company's overall advertising business grew at its fastest clip since before its May initial public offering, helping the company's revenue expand 40 percent and surpass Wall Street targets. Shares of Facebook were flat at $31.24 in after hours trading on Wednesday, regaining ground after falling more than 8 percent immediately after the numbers were released. "Overall solid quarter but maybe high expectations going into the quarter," said Aaron Kessler, an analyst with Raymond James. Facebook said its mobile business accounted for 23 percent of total ad revenue, compared to 14 percent in the third quarter. Some investors may have been expecting more from the mobile business. "Mobile revenue was expected to be a little higher," said Kessler, who said he was looking for mobile revenue to be 25 percent of total ad revenue. Finance Chief David Ebersman said that Facebook had "basically doubled" mobile ad revenue from the third quarter to the fourth quarter. "Two quarters ago we really had no mobile revenue," Ebersman told Reuters in an interview. In the course of a pretty short period of time, we've dramatically ramped up our ability to monetize mobile. The total number of monthly active users on the social network reached 1.06 billion at the end of last year, with 618 million daily active users, Facebook said. Facebook shares, which lost more than half their value following its May initial public offering, have regained ground in recent months as concerns about its mobile ad business and insider selling have eased. Shares have surged roughly 60 percent since mid-November. Facebook said net income in the fourth quarter was $64 million, or 3 cents a share, compared to $302 million, or 14 cents a share in the year-ago period. Excluding certain items, Facebook said it earned 17 cents a share, compared to the 15 cents a share expected by analysts polled by Thomson Reuters I/B/E/S. Facebook has ramped up its online advertising services in recent months, putting a greater emphasis on mobile ads and introducing capabilities that let marketers target Facebook users based on their Web browsing history. Advertising revenue in the fourth quarter grew 41 percent to $1.33 billion, with mobile ads representing 23 percent of Facebook's ad revenue. Facebook's overall fourth-quarter revenue was $1.585 billion, versus $1.131 billion in the year-ago period. Analysts were looking for revenue of $1.53 billion.