Facebook is now every where of our digital life. Its common for us to some time use a public PC or a friend or family PC or device (such as mobile phone) to access facebook and get connected with friend and family.
However many times it happens that we tends to forget to click the logout button and later repent about our account security.
To save users from abbuse, FaceBook as a nice feature to force logout any unauthorized activity from Facebook settings page.
You can access this feature from facebook account settings, Account > Account Security > Security.