Connecting to Facebook by logging into your Facebook account gives you access to social game features, such as: free lives via requests, competing against friends, playing across devices and saving your progress online.
Also, we can save your progress into our servers and restore it if you play while connected to Facebook. However, if you don't connect to Facebook or play the game offline, your progress will be saved locally (on the device) and will therefore be lost when the app is changed. We strongly recommend connecting to Facebook to ensure your progress is always saved.