"There is a bug where a_user was not cleared on logout. We will be fixing that today."

PR speak. This seems much less like a bug to me, more like a poorly contemplated decision.

What seems more likely is that they were contemplating what they should do, and given the general direction Facebook is headed, they probably thought little of just leaving it in.

I love the facebook guys, but calling it a bug as opposed to a non-malicious decision is just PR speak