Does Facebook Sign You Out When You're Idle?
Facebook offers many ways to protect your privacy and segregate what you share. You can specify exactly who can see each status update, photo album or Timeline post. If someone else has access to your Facebook account, though, everything is available to him. Signing out of Facebook when you're done with a session ...
There's also an extension for Firefox called Facebook Auto-Logout that logs users ... Some browsers also have add-ons that enable you to automatically log out ...