Per the SDK, I am using the logout.php with redirect:
https://www.facebook.com/logout.php?confirm=1&next={SOME URL}&access_token={ACCESSTOKEN}
This is one of the many annoying things about developing on the Facebook platform. The only way to log out via a browser session is to use their javascript logout function or the redirect like you tried but either will also log the user out of Facebook which is very annoying from a users perspective. The problem is you can't access the cookie they set for your app to delete it due to cross-domain restrictions so you are stuck with the limited options Facebook provides. If you are not prompting for offline_access, you could just forget the users access token and it will expire after an hour. Although the problem still exists that the cookie is still set for within that hour.