Please note Amazon's new virtual coins and other things like it are not technically currencies. The definition of currency:
The fact or quality of being generally accepted or in use.
It's unlikely Amazon's coins will ever become so widely accepted that you can go into any store or any marketplace and exchange them for goods and services, like you can with dollars or other traditional currencies.
The reason why is because they won't hold fairly consistent and widespread transferable value. The reason you can do this with dollars is dollars hold fairly consistent and widespread transferable value. Bitcoin doesn't have this yet, but in the future it may. The reason Bitcoin can gain that facet and other virtual currencies probably won't is because others are centrally controlled, but unlike being controlled by a central bank, which has the blessing and backing of government, they are controlled by private entities. You'll never get enough trust in valuing such a centralized non-government set up (what's to stop them counterfeiting/inflating?). Bitcoin, because it's decentralized, though, is different.