Something is wrong with Privacy settings, and the game has no rights to see friends from your friend list. To solve this use the recommendations below:
- Open your games list: https://www.facebook.com/games/manage
- Delete our game from the list. All your achievements will be saved.
- Start the game again and give the game permission to see your friends.
If your friends are still not displayed in your friend list in the game, your friends should do the same thing.