You can get the official “Log in with Facebook” button here. ZIP has PNGs for different sizes
https://developers.facebook.com/docs/facebook-login/checklist/#brandedlogin
EDIT 26/07/2014: Facebook seems to have removed those image assets. They now suggest you using a Javascript log in button with their button style. https://developers.facebook.com/docs/plugins/login-button
EDIT 21/02/2018: Facebook has updated the guidelines with a new “Continue on Facebook” button image asset
