It is important that you connect your game account with Facebook in order not to lose your progress when you delete the game. Currently, Facebook is the only way to retrieve account data.
To connect with Facebook;
1) Tap on Social in the Main Menu.
2) Tap on the "Connect with Facebook" button. The "+50 diamonds" icon indicates you will get 50 diamonds when you connect your account to Facebook.
3) Finally, tap on "Continue" to connect with Facebook. 
4) If you see your Facebook friends on your friends' list, you are successfully connected to Facebook. Your account data is safe as long as you stay connected to Facebook with the same account, and you can always retrieve your progress when you delete and reinstall the game.
Enjoy your 50-diamond prize!