I just added two contacts to the contacts app on my iPhone (iOS 9.1) and after each I had a dialog asking me to enter my Facebook password, presumably so it could cross-reference this with my Facebook account. I didn't enter my password and instead went looking for a setting that links the Contacts and Facebook apps together but could not find anything. Is there such a setting? If so, where? If not, is Contacts sending data to Facebook?
On iOS 9, if you go to the iPhone Settings app > Facebook (that's in the 7th "block" of settings as you scroll down, with social media accounts like Twitter, Flickr and Vimeo—whether you have an account or not), there is a disclaimer:
*Photos and user names are automatically updated for Facebook friends. Choose "Update All Contacts" to temporarily provide Facebook with email addresses and phone numbers from your contacts to update photos and user names for all matching contacts."
There is an "Update All Contacts" button. In addition, you can control there whether Contacts.app has access to your Facebook account (if you have one) on that same settings panel. You can control whether the Facebook app has access to your contacts through Settings > Privacy > Contacts (I think that's the same on iOS 8 and 9).