It's no secret that plenty of apps/services take your usage data and personalize your experience with it, but Facebook is trying something new with that information. The company is preparing to personalize its app's navigation bar with nav bar icons depending on how much it deems you need each one.
If you go to your Facebook app right now, you'll see five icons (in this order): News Feed, Watch, Marketplace, Notifications, and Menu. With Facebook's personalization, though, only the three essentials - News Feed, Notifications, and Menu - will remain, while the other spots are taken up by any of five other categories: your profile, friend requests, Marketplace, Groups, and Watch.
Interestingly, the amount of icons in your navigation bar will be variable from four to six icons for most. So if you only really look at your own profile but don't do anything else on Facebook, you should, in theory, only have four icons up there. On the other hand, if you use Marketplace to sell stuff, Watch to watch stuff, and Groups to talk about stuff, then you should have a pretty crowded nav bar with six icons.
Facebook has been testing this since February, but it'll roll the "redesign" out to everyone over the next few weeks. An alert will be sent when a new icon is added to the nav bar. You'll eventually have the option to select which icons you want to see in settings, but it doesn't sound like that'll be available at launch for whatever reason.