When you activate this option, Facebook will send you a text and/or email if a computer or mobile device (other than ones you register) logs on to you account. (Remember that when you surf the web, you're leaving a crumb trail for sites to see with your IP address and cookies). You can register multiple devices, and once you do, it puts an tighter layer of security on your account.
It's easy:
1. From any Facebook page, click the Account tab at the far right on the blue bar
2. In the drop down menu that appears, select Account Settings.
3. Scroll to the bottom of the Account Settings page, and you'll see an option for Account Security. Click the Change link next to it.
4. Once in Account Security, click the radio button to select Yes as the answer to Would you like to receive notifications for logins from new devices? and click the Submit button.
This feature is not activated until you:
5. Log out of your account (from the Account drop down menu)
6. Sign back in and go back to Account: Account Settings: Account Security.
7. You'll see a pop up asking you to name the device you're logging in from. Give it a name like you did when setting up your network.
8. Click the button to Save this device.
Repeat the register process from any device you use to sign on to Facebook.
I tested this by giving my password to a girlfriend and I was notified that moment she logged on. This gives you enough time to get to your computer, change your password and avoid possible disaster!
