If you don't see any friends who play Warzone at the bottom of the game screen, and/or when you try to invite friends, the list is empty, you need to reinstall the app with proper settings. Don't worry — you will not lose your base by deleting the game and then installing it back again. Please follow these instructions:
1. Go to the list of your Facebook games.
2. Find Warzone and click on the Settings icon:
3. Click "Remove App":
4. Click "Remove". DO NOT CHECK THE BOX "DELETE ALL YOUR ACTIVITIES".
5. Open Warzone.
6. Click "Edit the info you provide":
7. Make sure that the "Friend list" option is activated, then click "Play Now":