No, but I've done it to lots of people. Also, it's not a case of finding out someone's password and logging into their account, it's more a case of them leaving themselves logged in while they go take a piss or something, you must be swift ;]
Funny story, we have a lot (and I mean A LOT) of fire alarms living here in student halls and as you will all know, fire alarms mean routine evacuation asap. This means many students just leave their computers on and leave themselves logged into Facebook.
After we have all made our way outside, shivering and hilariously dressed, the security guards have to do a routine check of all open bedrooms/flats in the block. During this check it has become common knowledge that they will frape anyone foolish enough to leave themselves logged in.
'Tis amusing.