PayPal isn't a currency, it merely houses and transfers currency. BitCoin is a currency because the value is independent, or at least nonparallel, with government currency.
The value of a PayPal balance remains exactly the same value as whatever external currency it represents, but BitCoin value fluctuates based on speculation and, most importantly, on scarcity. In some ways, BitCoin is more of a true currency, in that way, than the US dollar, which is no longer tied to any real-world, scarce commodity.