They've restructured their staff, requiring all of their developers to have skills in mobile application development. Whereas previously the company had a small, dedicated staff that worked exclusively on mobile, now mobile is factored into every project. For example, the team which develops the Messenger service (you know, the Facebook instant messaging feature that is both really useful and often infuriating?) also has to develop for the mobile app version of Messenger at every step of the way.
The most important thing Facebook needs to tackle first is getting its mobile app working properly. If you own an iPhone, you've no doubt noticed the constant updates that keep coming out. As of late, they've all been for the better! Hopefully with time they can get the app moving fast enough that it's always an enjoyable experience.