Facebook's biggest challenge lies in mobile devices. Most Facebook users access it using a mobile phone or tablet computer, yet the 9-year-old company only started showing mobile ads about 9 months ago. Investors have been worried that Facebook isn't taking advantage of its growing mobile user base since before the company's initial public offering in May. Analysts said Wednesday's results show that it is on the right track.Facebook said it generated 23 percent, or $306 million, of advertising revenue from mobile. That's up from 14 percent or $153 million in the third quarter, the first time it disclosed such information. Facebook views the mobile space as its biggest opportunity, a point CEO Mark Zuckerberg sought to drive home during Facebook's conference call with analysts. "It allows us to reach more people, we have more engagement from the people we reach and I think we will be able to make more money for each minute people spend with us on ... mobile devices," he said. The inroads Facebook made in mobile advertising in the final half of last year impressed Bruno del Ama, CEO of Global X Funds, which owns about 50,000 shares of the company's stock. "We have been surprised about how aggressive they have been," he said. Facebook has been trying to squeeze in more mobile adverting without alienating users who are more interested in conversing with their friends than being subjected to a marketing blitz. The company appears to be striking the right balance so far, based on the number of people still regularly using the mobile apps, said Kessler of Raymond James. While Facebook's accelerated revenue growth is a positive sign, there's still a feeling that the company could be doing even more to mine revenue from its mobile audience, Kessler said. He expected Facebook's mobile ad revenue to rise to 25 percent of the company's ad sales or about $350 million in the fourth quarter.