When the keyboard is open, it is always in landscape mode even if you tilt the phone so that the screen is vertical. The only exception is the main screen of the Facebook app. Other apps and the home screens and other screens of the Facebook app is in landscape when the keyboard is open.
The Facebook app's main screen does not appear to be programmed to operate in landscape. It is in portrait regardless of orientation of the phone or whether the keyboard is opened or closed. This appears to be a bug or oversight in the Facebook app. Only the main screen of this app exhibits this behaviour. If you go into the news feeds of the FB app, it will switch to landscape orientation normally like everywhere else.
The orientation of the screen seems to be specific to applications (or home screen). Each application seems to be able to control the orientation based on the orientation of the phone and keyboard position. If the app is not programmed to change orientation, then it seems to be an issue of the app. So far, this one screen of this one app is the only one that doesn't behave as I expect it and I think it is just a bug in the app. Also, when I play Angry Birds, the game has no portrait mode. It's always in landscape.
thanks