I've just read the following advisory at http://www.securitypentest.com/2011/02/facebook-url-redirect-vulnerability.html. In its solution section, I found a strike-through text: "Only works when your login to Facebook". What's wrong with it? If it requires users to be logged in to exploit, so is it still a vulnerability?
Yes. I guess before that point, a user didn't even have to be logged in to be redirected through Facebook. But then, for some time, the user needed to be logged in to Facebook for it to work.
It is still a problem, since a quite a few people are always logged in to Facebook. Though even though a user might now have Facebook open right now, he might still be logged in, so the redirect system can still be abused for a fair share of people on the internet, depending on the audience. A login box won't apear, since Facebook will recognize the session cookie that gets sent to them.
The fact that it required user authentication didn't really reduce the risk much. IMO, The user most likely to be targeted by a vulnerability like this is the user that is going to be currently logged in and/or active on facebook.
The vulnerability has since been patched (or so it's stated), but yes, it was still a vulnerability when it required the user to be authenticated.