Facebook warns that shift to mobile devices could harm business For Facebook Inc., the No. 1 challenge is making money from mobile devices. And it's warning investors that so far its ad business is not keeping up with the shift to mobile devices, a crucial point as Facebook heads into the last stretch before its initial public stock offering. The Menlo Park, Calif., company's executives have been fielding questions about its mobile advertising strategy during the roadshow this week. Facebook flagged the potential slowdown in advertising growth in its latest filing with the Securities and Exchange Commission. More people are accessing Facebook from mobile devices, on which Facebook is showing them fewer ads. Facebook said it began showing mobile users "sponsored stories" ads in their news feed in March, but has limited the number of ads users see. The average time people spent on Facebook on their smartphones jumped to seven hours and 21 minutes in March compared with 6 hours and 31 minutes on personal computers, according to new research from ComScore Inc. Comscore has begun measuring mobile usage for Apple's iOS, Google'sAndroid and Research in Motion's BlackBerry on sites like Facebook. Facebook commands most of smartphone users' time, according to ComScore. Other popular services are Foursquare with 146 minutes, microblogging service Twitter with 114 minutes and Tumblr with 68 minutes. Getting serious about mobile was one of the major drivers behind Facebook's $1-billion play to buy mobile photo-sharing app Instagram. Facebook is seeking a $96-billion valuation in its IPO, largely because of how deeply it has knitted itself into people's lives. Facebook has about 158.9 million unique U.S. visitors who reach the site on computers and 78 million who reach it via mobile phones, although there is plenty of overlap between the two, ComScore found. Mark Zuckerberg skips Boston, so does IPO roadshow video