Mark Zuckerberg celebrated turning 28 yesterday on the final leg of a roadshow aimed at building demand for Facebook Inc.\'s initial public offering and convincing investors that he can make money from mobile users. Chief executive officer Zuckerberg, along with chief operating officer Sheryl Sandberg and chief financial officer David Ebersman, will meet potential shareholders in a cross-country tour that includes stops in Chicago and Denver. Facebook engineers, meantime, are putting final touches on products designed to help profit from the 488 million people who access the world\'s biggest social network on the go. Facebook said last week that growth in advertising sales isn\'t keeping pace with gains in users, many of them logging on from handheld devices, while some institutional investors have balked at buying Facebook stock, people with knowledge of the matter said. That adds to pressure on company executives to articulate their mobile strategy ahead of the IPO, due this week, said Jay Ritter, a professor at the University of Florida. \"In terms of Facebook\'s valuation, the big issue is whether they\'re going to be able to grow to have sustainable profits,\" said Ritter, who teaches finance in Gainesville. \"The biggest issue in that regard is the mix of desktop users versus people using their mobile device.\" Facebook plans to raise as much as $11.8 billion through the IPO, the biggest ever for an internet company. Its shares are set to price on Thursday and begin trading under the symbol FB on the Nasdaq Stock Market the following day. Facebook is offering 337.4 million shares at $28 to $35 apiece, and it\'s seeking a valuation of as high as $96 billion, filings show. The company plans to stop taking orders for the IPO after US markets close tomorrow, two days ahead of schedule, according to a person with knowledge of the transaction. Zuckerberg, at a meeting on Friday in Palo Alto, California, fielded questions about the company\'s mobile strategy, according to attendees who requested anonymity. Executives will probably get more inquiries on the matter at meetings in the coming days. Ads shown on mobile devices are smaller, harder to decipher and can be less likely to prompt users to make a purchase. That means they may be less appealing to advertisers and less lucrative than marketing messages on bigger computers. \"Growth in use of Facebook through our mobile products, where our ability to monetise is unproven, as a substitute for use on personal computers may negatively affect our revenue and financial results,\" Facebook says in its IPO filings. As part of its effort to wring sales from mobile users, Facebook unveiled a new online store last week to help it play catch-up with Apple Inc. and Google Inc. in the area of applications that can be downloaded onto handheld devices. Facebook has begun taking developer submissions for its App Center, a curated online store for programmes that run on the social network. Facebook would keep a cut of the revenue generated by developers who sell software in the store, helping it diversify away from traditional advertising. \"Wall Street cares about one thing: growth in profit,\" said Paul Deninger, a senior managing director at New York-based investment bank Evercore Partners Inc. In the early days of the road show, Facebook\'s IPO generated lower-than-expected demand from institutional investors who expressed concern about the company\'s growth prospects, people with knowledge of the matter said last week. The company also told analysts last week that sales may not meet their most optimistic projections, people said. In another move aimed at stepping up growth, Facebook said last week that it\'s testing a new type of ad, which lets users pay to have a status update linger on their friends\' news feeds. \"This particular test is simply to gauge people\'s interest in this method of sharing with their friends,\" Meredith Chin, a spokeswoman for Facebook, said in an e-mail. Currently, the test isn\'t running on mobile devices, she said. The paid status updates were initially reported by TechCrunch, a technology blog. Zuckerberg co-founded Facebook in 2004 with Harvard University classmates. While he has amassed more than 900 million users with easy-to-use tools that help people communicate online, his challenge is stemming slowing sales growth amid increasing competition from Google and Twitter Inc. Sales climbed 88 per cent to $3.71 billion last year. According to researcher EMarketer Inc., revenue may increase 64 per cent to $6.1 billion this year.