Official Facebook Connect button as PNG

Would comment on existing answer, but rep... =\

You can still download the icon here. I guess they've dedicated a separate domain to it now. I believe they will always have to provide you with an icon alternative since they require, just as google does:

The "f" Logo may be used alone or alongside other social icons. It should be used at equal size to neighboring icons. Be sure to maintain its shape and proportions and provide adequate clear space between logos.

You can't really do this consistently without a proper icon.

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