A good post at Bokardo about the Facebook logout button. The site isn't responding so I'm going to quote a good bit of the post.

At the very least, interfaces should not lie. They should not deceive the people who use them into thinking something is true when it actually isn’t.

Apparently, Facebook does not agree. On Sunday Nic Cubrilovic posted some troubling news: Logging out of Facebook is not Enough. Facebook doesn’t actually log you out when you ask it to. They pretend to, but they don’t. Instead, they simply change the status of your logged in session to fool you into thinking you’re logged out.

You don’t see your friends or profile. You don’t view your feed. Even if you try to access your profile pages, Facebook will send you to the login screen. Except that you’re not actually logged out. Every step of the way Facebook knows that it’s you trying to access those pages. You’re not really logged out, but Facebook is tricking you into thinking you are.

So, I’ve designed a more appropriate Facebook logout button…instead of saying “Logout” it now says “Logout (not really)”. This is more accurate and better reflects what’s actually going on.

This is a good example of the difference between a technical user and a non-technical user. For a non-technical user being logged out means not being able to access your stuff without logging back in, this is exactly what Facebook is doing.

To a technical user, logged out means that you no longer know who I am so I'm browsing the site anonymously.

This is an interesting exercise in language. I agree with the conclusion that this is bad for privacy, I think from the perspective of the average user the interface is truthful. 

What say you?