Not really a ponzi to me. Just don't invest anything and you will surely be not affected by any ponzi scheme. For faucets, you should do the research for yourself. Ask google if the faucet is still paying its users or you can check the micro earnings board to get some idea from which is the highest and best paying faucet in present time.

So , to answer your main question , faucet dispense free satoshis because owners are making a *little* profit from renting their spaces for ads. They are being paid for every total visit of their users.