Facebook "Sign-up" buttons.

Are you referring to the "Sign Up" button in the following screenshot?

If so, you probably cannot start the automatic authentication flow this way, but you should be able to set the URL to the Joomla Login page (with the JFBConnect buttons added using our 'Integrate with Joomla Login' option) or a custom article. You can easily add a {JFBCLogin} easy-tag to this article content, so that a "Login with Facebook" button is displayed in the article text.

If you're referring to something different, please provide more clarification.

Thanks
-Melissa