Here are some findings so far regarding this problem. The goal is to keep the phone contact list and Facebook completely separate. So far, what I've found is that other people are reporting that the Facebook contact list is being downloaded into the iPhone. I haven't seen any reports yet of the other direction, which is obviously the more troubling scenario. So far what I seem to be finding is that there is no way to prevent the contacts download, but once it has happened it is possible to either hide or delete the downloaded contacts. Also, this seems to be an iOS6 thing.
Here's a nice for dummies way to turn off FB access to contact list (this apparently does hiding). The comments are full of related suggestions, including some deleting ideas:
http://howto.cnet.com/8301-11310_39-57549165-285/how-to-remove-facebook-friends-from-your-iphones-contact-list/
Here's a cheerful page about how to get it to synch the contact lists, as if everybody would want such a thing. This is a good read for understanding how it is supposed to work, to help understand how to turn it off. There is a section on how to turn it off; the FB app apparently has a control panel for regulating FB access to phone features. Also the comments are full of people discussing ways to turn it off.
http://www.pcadvisor.co.uk/how-to/apple/3420848/how-sync-facebook-iphone-contacts/
Here it is for droid. This page was notable in that a really novice user needed things really spelled out.
http://www.droidxforums.com/forum/droid-x-help/41709-prevent-facebook-contacts-automatically-downloading-onto-droid-x2-contact-list.html
