They post codes to use on their twitter account too. These are referred to as Social Media Codes on the rules.
http://twitter.com/SubwayFreshBuzz
Codes from Twitter:
DLZT26L
DN1QTTU
Codes from Facebook:
CX91T8L (thanks to EllahJay88!)
CV812WM (thanks to PlumeNoir!)
I haven't seen any of...