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Why is the fractional-reserve banking not a Ponzi scheme? 
You are forgetting one crucial point regarding the money supply.  The US Federal Reserve increases the money supply, meaning some of the money is not really loaned, it just appears out of nowhere.  At first glance this seems even worse: over the short term, the Fed changes the money supply to help the economy in whatever way it sees fit.  But over the long term, the money supply increases to reflect economic growth.  As new technology is introduced, more can be accomplished with the same labor and resources, and thus the money supply needs to be increased.  Money is really just a convenient replacement for the barter system, so if there are more things to barter "for" (goods and services) then there should also be more things to barter "with" (money). Also keep in mind inflation.  The cost of goods and services goes up over time due to the inflation of currency, and so the money supply must also be increased so that those goods and services do not artificially increase in value, which would be very bad.