MLMs and Jewish Law
Rabbi Aryeh Wohlander (ph. spelling from the Hebrew), first distinguishes "pyramid MLMs" (where the money is mostly or exclusively from recruiting a downline, not from product sales) from "non-pyramid scheme MLMs". He notes that the latter are legal, but he distrusts 'we are not a pyramid' claims of MLMS (he notes all MLMs make them) and gives a rule of thumb: whether one can make money without recruiting others or having a downline.
In practice -- I add, the Rabbi does not, perhaps due to Ynet's fear of lawsuits -- this means all MLMs are pyramid schemes. As for pyramid schemes, whether with a product ("pyramid MLMs") or not, the Rabbi notes that it is explicitly forbidden -- a form of fraud -- to give a false description of the product (in this case the MLM "opportunity") or to hide flaws in it, e.g. by overemphasizing atypical success stories or blaming failure on the person instead of the rigged system.
What's more, since a person who joins and does know all the facts, and then does not succeed in recruiting others had lost money due to deliberately bad advice by the recruiter, this is a serious sin, hurting others who are in no position to defend themselves, forbidden by the biblical command to 'not put an obstacle in front of a blind man'.
Finally, such "pyramid MLMs" are similar to gambling, an activity that 'is of no use' -- one man's gain being simply another man's loss, which is forbidden by Jewish law (e.g., by the Rambam), even if there is no fraud involved, as a waste of time, to say nothing of its corrupting properties. This is as opposed to genuine economic activity -- buying or selling goods or services -- where both sides gain from the deal, and thus is not a waste of time.