Here are parameters posted by the FTC (Federal Trade Commission):

"Multilevel marketing plans, also known as "network" or "matrix" marketing, are a way of selling goods or services through distributors. These plans typically promise that if you sign up as a distributor, you will receive commissions -- for both your sales of the plan's goods or services and those of other people you recruit to join the distributors. Multilevel marketing plans usually promise to pay commissions through two or more levels of recruits, known as the distributor's "downline."

If you are committed and you devote enough time to develop your "business" to make it successful, MLM CAN and probably WILL make you a millionaire.  MLM is probably the easiest and fastest way to make a lot of money when you are successful (unless you win the lottery or you are a professional athlete of course!).  But don't fool yourself, it won't happen tomorrow.  Like any other business, MLM takes time before becoming fruitful.  You just need to find the one that is right for you, one that will teach you how to get there and support you along the way.  And once you have found it, don’t quit. The only way to fail is if you give up.   Be patient.  Big incomes never happen overnight.  Think BIG!  Small desire produces small results just as small fires produce little heat.