(Telecompaper) Facebook reported fourth-quarter sales up 63 percent from a year earlier to USD 1.59 billion. Ad revenue rose 76 percent to USD 2.34 billion, of which 53 percent came from mobile services, more than double the share a year earlier. Despite a strong increase in staff and infrastructure costs, Facebook saw its operating profit double year-on-year, to USD 1.13 billion from USD 523 million. The operating margin improved to 44 percent from 33 percent in Q4 2012. Net profit jumped to USD 523 million or 20 cents a share, from USD 64 million or 3 cents a share a year ago. The company generated free cash flow of USD 2.85 billion over the full year and ended 2013 with total cash of USD 11.45 billion. Facebook a further rise in daily active users on the social network, to an average 757 million in December 2013, an increase of 22 percent year-over-year. Mobile daily active users rose 49 percent to 556 million. On a monthly basis, the site had 1.23 billion average users, an increase of 16 percent, and 945 million mobile users, up 39 percent year-over-year.