Someone else writes:

        Pyramid schemes, although not illegal unless conducted by mail, are
        scams *IF* they promise to bring you vast monetary rewards. It is
        not impossible, but highly improbable that you would make any
        decent money from such a scheme.

From : Jim Steigelmann

Actually, there exist pyramid schemes which are illegal, even when not
conducted by mail.  A good example was one that went around when I was
in college called "airplane".  In this "airplane", you bought yourself
in as a "passenger" for $2,000.  On an "airplane" there was one captain,
two co-captains, 4 flight attendants, and 8 passengers.  The 8 passengers
would pay $2,000 each to the captain, who would walk off into the sunset.
Then everyone would move up one seat.  The co-captains would each be
promoted to being "captain" of their own airplane, the flight attendants would
become co-pilots, and the passengers would become flight attendants, and
everyone would be out to recruit more "passengers" to continue the game.

This was, in fact, illegal, and was prosecuted as fruad.

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Jim Steigelmann