Well, that's the reason why a lot of people say that BTC is a Ponzi scheme. But the fact that people bought something of limited supply using real currency made Bitcoin worth something. Come to think of it - what is gold tied to? What if one day people say that it's just a piece of metal and they don't want to value it? Everything that people consider and want to be valuable becomes so, in case of gold it's beauty, durability and limited amount, in case of Bitcoin - it's very convenient and innovative technology. Another thing that matters is that Bitcoins don't come out of nothing (like everything in Ponzi schemes) - they are mined and mining requires electricity power, which has a value. And the more there are Bitcoins out there - the more power it is needed to mine some, this genius concept practically eliminates the possibility of inflation.