This is when go on to Facebook (or My Space, Bebo etc) under a friend's user name because they've been silly enough to keep their account logged on unattended. Once you're on, to punish them for making this silly mistake, you then message random people random things, changing their status etc. (nothing offensive obviously). Once the dirty deed is done, log off leaving them none the wiser.
'I went went on facebook on a pal's computer, I forgot to log off and got facebook-raped!'