Facebook imports and share your entire mobile contact list in to your Phonebook on Facebook. Not sure how you may feel about it but your friends that are not on Facebook and does not list their mobile telephone number might throw a hissy fit. #justsaying you might want to do some housekeeping from time to time.
Follow the instructions to remove the syncing. Your Facebook friends that share their mobile telephone numbers will remain on your contact list.
Click the right hand corner Account>click edit friends>click contact