You can use faucets for free, so they aren`t Ponzi schemes (but you will earn low satoshis rewards). If the faucet gives you the possibilite to invest, you must see if their offer isn`t too good to be true. Otherside it`s a "hybrid'' faucet/ponzi scheme. So don`t invest.

Instantly payment faucets are the best to use also. So you don`t need to worry about if you will be paid or not.