I think the difference is: iPhoto says "This is a face. Would you like to type in who it is?" And Alice would type in "Bob". Whereas Facebook uses Bob's profi

I think the difference is: iPhoto says "This is a face. Would you like to type in who it is?" And Alice would type in "Bob". Whereas Facebook uses Bob's profile information to say "Is this a photo of Bob?", leaking Bob's identity to Alice.