TLDR: With 500 million people using them, Facebook Credits could become a viable alternative currency to the dollar, yuan, etc. Especially if FB starts entering deals that allow FB currency to be used to purchase real world goods and services, say at Walmart, and/or deals that allow FB users to send each other 'moneygrams' or other payments worldwide using FB currency. Proof of concept has already happened with at least one popular virtual currency in China.

Lotta speculation, but very interesting to anyone interested in how money works, or international economics.