You can link your account to online services as follows:

1. Open the Menu and choose Account
2. Tap the "Link current account" button
3. Select a service and tap the "Link" button
4. Enter the details of your Facebook or Google Play/Game Center account

Linking your game progress to a service (Facebook, Google Play/Game Center) means that one game account can be linked to one service. If you wish to link a second game account to the service, this will require replacing previous game progress with current game progress. This means that the previous game progress data will be lost forever without any chance of recovery.

Take care not to replace and lose the game accounts that are important to you. If you're not sure about what you're doing, you should link your account to another service.