Further down the Settings list, you'll see separate settings for Facebook. These let you download and install the software as well as sign in with a specific account. One of the coolest options here is to update your contacts with Facebook data, including profile pictures. To do this, simply tap the Update contacts button. Depending on the speed of your Internet connection and the number of contacts needing an update, you should whip through this pretty quickly.
You'll also notice that Facebook now gives you the option to not only work with your contacts and your calendar but also interfaces directly with iTunes. Apple is phasing out Ping, its social network inside the Apple Music Store, and seems to be replacing it with Facebook, at least on your iDevice.