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Facebook already supports this (as another commenter says) but you can also send a text to FBOOK with the next "otp" to get a one-time password that expires after a few minutes.

If you're in a place where it's likely your computer could be compromised then this keeps your regular password secure.

Allowing a quick out-of-band reply to invalidate a login from a new device is a great idea.

You can kinda do this, but the current implementation is cleverer and only triggers for logins from new computers. The option is in the same place where you enable https.