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 aft

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.