I just tried it and it seemed to work fine for me. I'm wondering two things: (1) at what point in the process did you get this error? At redirect? (2) Are you running on localhost:8080 or some other host/post? I ask this because OAuth 2 is sensitive to the callback URL sent on the initial redirect being the same as what is registered on FB with the application...if they're different, you'll run into trouble. As it is, Spring Social Showcase is registered with FB such that the callback URL must be based at http://localhost:8080/spring-social-showcase/.
Craig Walls
Spring Social Project Lead