My Free Farm 2 has no interaction with Facebook, so it's not possible to link your account to Facebook.
If you're looking for a way to keep your account safe if you should lose access to your account, you can set a username and password in the settings/farmhouse using the "Set Password" button. That way, you'll be able to log into your account from other devices.
If you're looking for friends, you can use the "Suggest Friends" feature. Just click on an empty friend slot in the friendship menu and use the "Suggest Friends" button to get a list of players who are also looking for friends.