Your friends list should sync up automatically if any of your friends are playing the game and are signed in with their Facebook accounts.
There might have been an issues with retrieving your friends list when you logged in, but he game will retrieve your friends list again after 24 hours - so please leave your game running in the background of your device for that length of time to see your friends appear.
Also it's possible that you refused the permission to allow us to retrieve your friends list the first time you connected your city through Facebook. If this is the case, please:
1) Make sure you're logged out of Facebook in the game
2) Delete the existing permissions by visiting your Facebook account and clicking 'Settings' > 'Apps' > Then hover over 'SimCity BuildIt' to see the 'Remove' option appear.
3) Remove the app (please note, this will not delete your saved city!)
3) Go back into SimCity BuildIt on your device, then re-connect to Facebook. The permissions will then reappear.
