More and more people are saying that you should log out of Facebook as soon as you are done. If you stay logged in and go to another site with which Facebook has an information-sharing agreement, Facebook will tell the other site all sorts of things about you that you thought were private. (You can “opt out” of this, but the procedure may be too complicated for most mortals.)
But have you noticed that Facebook is the only site on the Web that requires you to log in, but does not give you a simple one-click “Log Out” link on each page? What you have to do is first click “Account”, then click “Logout”, which is much less obvious.