2 answers
I talked with a couple of Salesforce's SSO sso gurus. They said, and I quote, that this scenario "works great." Right. If you enable both, what you should end up with is a login page with username/password fields and a 'Login with Facebook' button. Entering username/password would cause Salesforce to verify them against the configured SOAP service, while clicking the button would redirect you to FB for authentication.Curious as to the use case here... Are you considering authenticating internal users against FB, or is this for a community?