this is by design, having your app in landscape and login flow in portrait settings cause some problems with rendering - but it looks like you want to use landscape orientation for login as well
you can set it up as you like in android manifest:
<activity
android:name="com.facebook.LoginActivity"
android:configChanges="keyboardHidden|orientation"
android:theme="@android:style/Theme.Translucent.NoTitleBar.Fullscreen">
</activity>
android:theme is Translucent - meaning you game will be visible under login dialog
<activity
android:name="com.facebook.LoginActivity"
android:configChanges="keyboardHidden|orientation"
android:theme="@android:style/Theme.NoTitleBar.Fullscreen">
</activity>
android:theme isn't Translucent - login dialog has black background