The definition of insanity is repeating the same mistakes over and over again and expecting different resultsHowever, the quote had been mis-attributed to a variety of people including Albert Einstein, Mark Twain, Ben Franklin, and others. However, its real origin is from a Narcotics Anonymous newsletter dated 1981 (scroll to page 11). Which means this is a quote about addiction.
Which makes it oddly appropriate about MLMers, specifically, about SERIAL MLMers, those people who keep joining one MLM after another, looking for "success" and kept spending money on the products, on his or her upline's advice, always having an excuse (the leader made a mistake, the market wasn't ready for us...) As they keep doing the same thing over and over, hoping for a different result.
One such example was the "confessimonial" on SaltyDroid... a story by "Roger Wilco". RW described a relationship with a woman who was heavily into MLM, spent well over 200K into various MLMs over 7 or so years, and in the end, chose MLM instead of him, never finding any success.
Addicted people, by certain "colloquial" definition, are "insane" because they are driven by their addiction into making choices that no "ordinary" people would make.
And MLMers, similarly, are "addicted" to MLM that they make choices that no "ordinary" people would make. That is a form of insanity.