Ponzi is where the payment that you get is from other's money. So in ponzi you should invest first so that you can assure that you will get paid because they get money to new investors. In faucet they give you few satoshi but it's like give and take because you take satoshi from their sites but you also give them chance to earn by visiting their sites and they earn money by the advertisements that are attached to their site. I hope you got my point on comparing faucet and ponzi  .