As a convenience, Sonlet allows you to use your Facebook account for logging in.  Note that using Facebook for login is not required, it's simply provided as a convenience for those that prefer it.

If you already have a Sonlet account that you'd like to connect to your Facebook login, just head on over to your account Settings. 

 Scroll to the bottom of your General Settings and click Social Account Connections.

Now click the Facebook "Log In" button.  It will look something like this:

It will ask you to log in to Facebook (if you're not logged in already) and to authorize the Sonlet application:

Click "Continue as <Your Name>" and you should be all set!  You will now be able to log in to Sonlet by clicking the Facebook "Log In" button, which is often faster and more convenient than typing in your password.