Facebook allows you to remotely terminate your active sessions from any terminals (smartphones, PC or tablets) at any moment. So if you are still logged-in on Facebook on another PC /terminal, here's how to sign-out in a few clicks:
Login to Facebook.
Click on Account settings > General
In the right pane, expand the "Active Sessions" section
As in the figure below, you shall have your"Current Session" and the other active sessions.
Simply click on "End activity" next to the session of your choice.