Hi! I know I've done this once or twice on my phones and I just had no clue how I had managed to do it... But I'll try to help!
- check your contact settings - see if there's any "sync with other accounts" option
- go to your Facebook app settings and see if there's anything there that can be synced with your contacts
- if all else fails, see if there's anything in your Settings app that can help you - look for anything with regards to third-party apps
Good luck!