A situation would be to have forgotten to disconnect from your Facebook account, and a person who will use the computer after you can easily return to our profile and make changes to the account or to change your password.
Another possible scenario would be to be connected to the Wi-Fi can easily take your Facebook logon information (eg add - onul Fireship Firefox), in this case I recommend you immediately change your Facebook password.
So these beings said you should check these on Facebook and take action if you have not logged in you account.
Mail alerts when logging on to Facebook to make it easy. Come on your account, join Facebook Account in the upper right corner then click Account Settings in the right here click Show Account Security
Check Send Me An Email , you will receive a mailde now every time you log on you or someone else's Facebook account.
Mail you receive will look like
If you set the IP at work and home or telephone, when I log on those computers will not receive alerts. If you find something suspicious you can access the link - and you can change the password received.