Ah, I remember writing about this years ago (2010 it turns out) - when they were very explicit about it from what I could see in my app at the time - http://caiustheory.com/facebook-iphone-app-contact-sync-isnt-automatic/

I hadn’t thought about that for a while though, and it seems they haven’t automatically uploaded any contacts against my account at least. Pretty horrible that people can end up with contacts synced that they don’t know about or understand has happened though. Just one more thing to watch out for still whilst using Facebook (and other social networks) irritatingly.