Friends are a great way to enjoy additional features of the game! Such as helping and challenging each other.
If you're having trouble seeing your friends:
If you're still having problems, try checking the privacy setting for your Facebook profile. Make sure permissions for the game are not expired.
You can also try removing the game from Facebook, and re-adding it. Doing so shouldn't delete your progress.
Please note that friends might not show up if they haven't been active in the past 48 hours.