You have to hand it to AmericanExpress for sheer brand chutzpah. They commandeered the idea of Social Currency and seem to be doing a good job (or at least a better job than their counterparts) of leveraging the social space to build onto their existing cardholder relationships.
Remember back to the early days of Twitter, (just after we all figured out what it was) when marketers discovered that this new channel might actually have some commercial value? It seemed like the very next....