Facebook gave potential investors something more to worry about yesterday than Mark Zuckerberg’s hoodie. According to its amended S-1, Facebook is seeing a shift in how users access the site from the desktop client to mobile devices. Unfortunately, the company said, “We do not currently directly generate any meaningful revenue from the use of Facebook mobile products, and our ability to do so successfully is unproven.” The problem is that the social network is unable to serve up as many ads to mobile users as it can to those using a desktop, which means its ability to monetize its user base through advertising is eroding. At the same time, Facebook announced a new app store to make it easier for users to find and access apps, which could eventually emerge as a new profit center to supplement ad revenue. In any case, it’s yet another reminder to investors that Facebook’s business model is still very much a work in progress.