There's been some discussion on what is the failure rate in MLM. Nobody can give a blanket answer because there are various MLMs and certain MLM, such as Pampered Chef, actually focuses on selling products while other MLMs might focus on self consumption.
But there are some tidbits that can be found that give clues about the failure rate in some MLMs. To be fair, a definition of failure should be made.

For the purpose of this thread, I'll say a sustainable net profit would be considered a success. I believe that someone who makes a net profit is likely to stick around while people who lose money, tend to leave. People who sign up and do nothing - I would consider a failure. Someone who signs up only to buy products would not be a failure but we have no verifiable information on what this number is. The attrition rate would suggest it is not that high, although it may vary depending on which MLM they are in.

These are from Wikipedia:

The Times: "The Government investigation claims to have revealed that just 10% of Amway's agents in Britain make any profit, with less than one in ten selling a single item of the group's products."[29]

Eric Scheibeler, a high level "Emerald" Amway member: "UK Justice Norris found in 2008 that out of an IBO [Independent Business Owners] population of 33,000, 'only about 90 made sufficient incomes to cover the costs of actively building their business.' That's a 99.7percent loss rate for investors."[30]

Newsweek: based on Mona Vie's own 2007 income disclosure statement "fewer than 1 percent qualified for commissions and of those, only 10 percent made more than $100 a week."[31]
A lot about success or failure rates are widely unknown because MLM companies won't release that kind of information (if they even possess that information). We do know that some companies like Herbalife, has a 90% attrition rate. The income disclosures (gross income) would suggest that some IBOs make high incomes but they are statistical outliers while the rank and file IBOs are very likely to be losing money, especially if they are participating in some training system.

While many people may criticize the research and theories of some anti MLM advocates, I have yet to see any comprehensive research from MLM advocates to counter whatever research is available. Let the discussion begin.