It's not about number of users, it's about liquidity for the money/currency in question (whether it be bitcoins, litecoins or "ethers").
Ie. how much will selling/buying a certain amount of the currency affect the market price. This is what defines money: high liquidity (market depth). Something cannot be money without this property. It might have the potential to become money, though. But without liquidity it is as much money as baseball cards and wine.
Atm Bitcoin still seems the only safe bet for the long run.