Not all MLM companies are created equal. Many see an initial burst of success followed by a gradual tapering off of profits, causing them to collapse and go out of business. MLM companies that succeed have sound business models, both for those who run the company and for those who sell product and recruit new sales agents. There are many sites devoted to MLM rankings, creating lists of companies likely to provide a return on investment to sales agents interested in the industry.
Multi-level marketing is a legitimate business strategy, though it is controversial. One problem is pyramid schemes, which use money from new recruits to pay the people at the top, often take advantage of people by pretending to be engaged in legitimate multi-level marketing. You can spot pyramid schemes by their greater focus on recruitment than on product sales.