This redesign will see Facebook start to personalize the navigation from user to user based on the features/services that they access frequently on Facebook. For example right now in the US, the navigation bar (bottom row for iOS, top of the screen for Android) consists of News Feed, Notifications, Menu, Watch, and Marketplace.
However in the future, these icons will change based on what you use the most. The initial rollout will see these icons change automatically, but Facebook states that eventually users will be able to change what they see and have more control over it. The number of icons will also vary and range between 4 to 6 icons, depending on your usage.
According to Product Manager Alexa Andrzejewski, “We want the shortcuts available in the navigation bar to reflect how an individual uses Facebook. To that end, we’re customizing mobile shortcuts so everyone can more easily connect with their most frequently used products.” These changes are expected to roll out in the coming weeks so don’t be surprised if your Facebook app’s navigation bar looks different.