This would require a valid access token and a valid redirect url.
Alternatively this can be achieved client side by calling FB.logout() .
In practice though there is a third (obviously not documented) unofficial way to achieve the same.
It is the case indeed that the standard log out form of Facebook suffer from CSRF.
Indeed while posting to http://www.facebook.com/
According to Facebook (I have been in touch with them about this) this is not a security issue though (primarily because the lack of same-origin enforcement on setting and clearing cookies).
Here some collected thoughts from the Google security team about this topic :