Schoolboy error to leave your facebook logged in, or even your computer in some cases, especially around friends houses...
In my Saved passwords/emails over Opera, Firefox and Safari I have 5 peoples details saved from where they clicked the "remember my password" button... Just waiting for the day any of them decide to have a birthday, or do something to insult me, I feel so nerdily-powerful.
And yes, facebook rape is your own fault if you leave it logged in.
Somebody once changed a friends relationsship status from "in a relationship" to "single" ... As you can guess, that didn't go down best well.