As of July 2013, currency in circulationthat is, U.S. coins and paper currency in the hands of the publictotaled about $1.2 trillion dollars. The amount of cash in circulation has risen rapidly in recent decades and much of the increase has been caused by demand from abroad.
So we still have a long way to go to supersede fiat currency. The thing is, fiat cash is being printed like toilet paper and it decrease in value, where Bitcoin increase in value. ^smile^
Dollar has it owns circulating stocks in the market or in transaction. So it's system is quit different compare to bitcoin. We all know bitcoin as a form of crypto-currency which its stock is based on the web and it's value depends on the transaction on the block chain or the support of the people choosing bitcoin as a form of transaction. While fiat mainly base its value on the given value implemented by the government that somehow is stable. And Bitcoin depends on fiat more than fiat depends on crypto-currencies.