On iOS, i installed the Phonegap Facebook Connect plugin but that doesn’t use the native Facebook app to perform Single Sign On. That opens the standard dialog :
I tried with a new project, without anything else that the default content, that works.
My main project’s setup seems be bad with the Phonegap Facebook Plugin, probably because of this : Issue with Facebook plugin on iOS.
I will clean my main project and try again, by removing and installing again the plugins. That should be ok