Probably because the App logs in differently the OS is my guess. Twitter and Facebook authenticate in different ways and their integration into iOS will differ. Perhaps the login on iOS for Facebook was targeted toward the older version of Facebook for iOS.
But then I'm thinking that an update should be coming to the app. The Facebook app currently also has a bug where the sound isn't coming through on notifications that started about a couple weeks ago that is being reported everywhere.
None of the iOS 6 API calls can be made in apps until the 6.0 SDK apps are available in the App Store - which will follow the public release of iOS 6 and delivery of iPhone 5 hardware.
None of the iOS 6 API calls can be made in apps until the 6.0 SDK apps are available in the App Store - which will follow the public release of iOS 6 and delivery of iPhone 5 hardware.