It is possible that you did not give Pet Master the necessary permission to access your Facebook friends’ list. Sometimes these permissions are revoked after an update and must be re-approved.
Please follow these instructions on your mobile device:
1. Close the Pet Master app completely
2. Open the Facebook app on your mobile device
3. Click the drop-down menu. (from the upper right if on Android, from the lower right if on iOS) then select "Settings and Privacy."
4. Select "Settings" from the lower part of the screen
5. Select "Apps and websites"
6. Select "Logged with Facebook" and click on the Edit button
7. Select on the Pet Master app then click "Remove"
8. When asked, Remove Pet Master? click "Remove" again
9. Restart Pet Master click "Allow" on the permissions notice
10. Click "Continue" when told "Pet Master" wants to use "facebook.com" to Sign in
11. Click "Edit This" and make sure the Friends List option is enabled
12. Click on "continue as _name_". - This will bring you back to the Pet Master game and you can continue to play as normal