After you have connected to Facebook, you can open the game again from anywhere.

 Go to the Settings section of the game (a gear icon below you Energy and Level totals). Tap "Connect," a blue button with a Facebook logo, and you will be prompted with the "Choose a Saved Game" window if you have a saved game somewhere. 

Choosing "In the Cloud"
will immediately load up your game that you last connected to Facebook with. Note that you can only have ONE game saved to the cloud, so choosing this option will immediately overwrite your saved game data on that device so far. 

Choosing "On this Device" will connect your current game on your device to your Facebook account, so your previous game data on Facebook will be deleted permanently.