To reclaim its status as a $100 billion company, Facebook Inc. is re-engineering itself into a mobile business.

Such a transformation has been daunting for one of the world's most-trafficked websites. Just over a year ago, Facebook's mobile business barely existed. Less than two dozen engineers were working on mobile applications as the social network focused on growing its desktop business to a billion users. While Facebook was a popular mobile app on Apple Inc.'s iPhones, users complained the software was slow and frequently crashed.

All of that began to shift in late 2011 when Facebook Chief Executive Mark Zuckerberg realized he had miscalculated. Facebook's own data showed that a large number of users