People who doubt the viability of emergent currencies always for a successful example. Larry White and others have given some examples, but they tend to be small, short-lived, and have catastrophic ends. As you know, there are two motives for holding a currency, the transaction motive and the speculative motive. Yes, dollars depreciate, but they are very low transaction cost for buying things. Bitcoin values fluctuate so wildly that they add an additional cost, because we aren't sure of the value tomorrow. Some Latin American countries have solved this problem with UF's, or unidad de fomento, a fixed amount of (in the case of Chile) US dollars. Of course, that means that the value of the currency is still tied to a fiat currency. So, I think the answer to your question is that no private money has, to date, BOTH survived for very long AND had a stable value that would allow its use for transactions.