Hi, just wanted to pass this along as one of my Facebook friends brought it to my attention the other day.
In Facebook if you go to Account-->Account Settings-->Account Security you will see that it says "To help keep your Facebook account as safe as possible, we can notify you when your account is accessed from a computer or mobile device that you haven't used before.
Would you like to receive notifications for logins from new devices?"
Choose yes, that way you are alerted if someone else tries to login to your account. This post from Mashable tells you exactly how to use it: http://mashable.com/2010/05/13/faceb...profile-steps/
Also, it's a good idea to change your password sometimes too--once a month if you can.