As hinted by Tim Cook, Facebook is coming in a big way to iOS. Just like Twitter, Facebook will be integrated into iOS 6 with a single sign-in that's shared across apps. You can use this feature to share images from iPhoto, websites from Safari and locations from Maps. It'll be a public API so other apps can tie into this feature as well.
Not surprisingly, the Facebook integration will let you sync your Facebook contacts with your address book on your phone and your Facebook events with your calendar. It's very similar to the Facebook experience on Windows Phone. A similar integration will also be available on the Mac.