In the app when you tap on the "Facebook" button to login, you are directed to Facebook’s login platform. Each time you go into Sync.ME the app attempts to login auto matically to your Facebook account.
You might see a small message "Can’t re-login to Facebook" if it can’t login successfully (which can be likely due to Facebook). In this case, if you have the Facebook app on your device, you should check your account login there. You can also logout of your account from the Facebook app, and then go back into Sync.ME to login.