"We want the shortcuts available in the navigation bar to reflect how an individual uses Facebook," said product manager Alexa Andrzejewski. "To that end, we're customizing mobile shortcuts so everyone can more easily connect with their most frequently used products, such as Profile, Marketplace, Groups, Watch, and Friend Requests."
The bar, located at the bottom of iPhone screens and the top of Android phones, will replace the default menu you see now (News, Watch, Marketplace, Notifications and Menu in the US). You'll still see news, notifications and the menu icons, while the other two slots will update to show you the items you use more than others. The more you use a shortcut, said Facebook, the more likely it will show up in your main navigation bar. Anything not in the main bar will still be available via the Menu icon. Eventually, Facebook wants to add controls for you to customize which icons you see, rather than just automatically setting them according to usage.