Spring Social's Facebook API binding uses Facebook's Graph API and there is no way that I'm aware of that you can logout a user via the Graph API. Likewise with Twitter and other providers However...
You can accomplish Facebook logout by redirecting to the following URL:
So, after the user is logged out of your app, you could follow that up with a redirect to that FB URL.
I'm not sure that this is necessarily a good thing to do, however. I would think that most users would be a bit upset to find out that by logging out of your app that they are also logged out of Facebook.
Spring Social Project Lead