My point is that data portability should be transitive. If by manually entering a friend's email address I 'own' that data, and it is thus valid for me to export, any service that imports that data should also allow exporting it. Otherwise it's lossy if I want to move on to another service.

In the FB case it's perfectly reasonable to prevent me from exporting data that I didn't provide, especially if another user provided it and has set privacy controls around it.