These days hackers and some other are so smart that may steal your account passwords and you never get to know about it, as they keep on logging to your account from other computers without your knowledge and peek into your account without your permission.
In order to get rid to this worry that whether some one else is using your Facebook account, you should better enable the security option in Facebook which allows you to register your computers and mobile phone from where you access your Facebook account.
So, if any one else login to your account from some other computer other than the registered one or the some other mobile phone other than your registered mobile phone you will get notified via email.
Follow the procedure below which shows how to enable this security feature in Facebook
1. Login to Facebook in any Internet browser
2. Go to Facebook Account Settings
3. Locate Account Security under Settings and click Change and the check yes to enable this feature and click submit button.
4. Next time you login to Facebook it will ask you register your computer, you will need to enter your PC name.
Note: Now when someone else login to your account, you will get notified via email and via SMS also if you are subscribed for it.