Mariano said that young people have been drawn to the flexible work schedule and ability to use social media and other new media as marketing tools for the products. Meanwhile, critic Robert FitzPatrick, who runs the website Pyramidschemealert.org, attributed the change to the rough jobs climate for college students and recent graduates.
How much money do they actually make?
In a promotional video on its website, Herbalife offers potential salespeople "an unparalleled international business opportunity."
But in reality, the majority of industry salespeople make modest monthly incomes -- if they make any money at all. Some sellers may even lose money after their upfront expenses and purchases are taken into account.
In 2011, about 25% of Herbalife's salespeople achieved so-called "leader status," qualifying them for commission checks. Of these top sellers, active leaders received a median annual income of $637. Meanwhile, the top 0.6% had a median compensation of $336,901, according to company filings.
Other companies tell similar stories. In 2011, only 12.7% of Nu Skin's 80,613 active salespeople in the U.S. received a commission check. And more than half of the $114.2 million in U.S. commissions it distributed that year went to less than 1% of salespeople, a group of roughly 113 salespeople dubbed "Blue Diamond Executives."
Many companies, like Avon and Mary Kay, released few details on the average income of their sellers.
How many of these companies are there?
In the U.S. alone, there are hundreds of multilevel marketing companies that sell a variety of goods and services.
The Direct Selling Association currently represents about 200 companies and says there are about 50 more awaiting admission.
Many U.S.-based companies also have massive international sales operations that in some cases make up the majority of their sales.
What do they sell?
Pretty much everything. Many of the household names in the multilevel marketing industry, like Avon, Mary Kay and Nu Skin, sell cosmetics and skincare products. However, there are companies that sell candles, groceries, insurance policies, electrical service, you name it.
Amway -- which has been around for decades -- sells food, vitamins, cookware and makeup, among other items.
Related: Nu Skin and the short-sellers
Wellness products, such as energy drinks or the weight loss bars and shakes sold by Herbalife, have been the latest craze.
Take Beachbody, the company behind Tony Horton's fitness craze P90X.
In the two years after Beachbody created a multilevel marketing division using "Team Beachbody coaches," its sales spiked more than 60%.
So is it worth it?
Critics say that the multilevel marketing model is unsustainable. If you recruit all of your friends and neighbors to sell the same product, they ask, won't you eventually run out of customers?