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By the time someone hacks into your Facebook account and sends all of your friends plaintive messages about being mugged in London, it's too late to do anything about it. However, Facebook does have an early-warning system of sorts. Using a security setting, you can have the service alert you whenever your account is accessed from another location, giving you a chance to (hopefully) force the intruder out and change your password.

The somewhat buried feature (it's under Account Settings, not Privacy Settings) lets you register your computer, and any other devices you use to access Facebook. After that, any time someone logs into your account via an unauthorized device, you'll get an email and/or text message. Of course, it would be even better if you could simply block access from any unauthorized computer, but then you'd never be able to check your wall from a friend's house or borrowed phone, and who wants to live with limitations like that?

source: http://consumerist.com/2010/05/facebook-can-warn-you-when-someone-else-logs-into-your-account.html