If you forget to log out of Facebook at the office or at a friend's house, don't worry. From "Account" choose "Account Settings," then "Account Security." Here, you can choose to be notified via SMS or email if a new computer or mobile device logs into your account.
Another valuable feature: You can also view the latest activity, including the time, location and device that accessed your account. If any of these locations seem suspicious, you can choose "end activity" to log out of them.