You can connect your game to your Facebook account from the game settings. Tap on the cogwheel at the top right of the screen, and then tap the Connect button with the Facebook icon on it. You will then be prompted to log in to your Facebook account.
Once you have connected your game to your Facebook account, your game progress will be saved to that account. If you ever need to reinstall the game, or wish to play on more than one device, you can restore your progress by connecting to the same account.