"Besides the Madoff saga, Marquet International, a consultancy, has identified more than 300 sizeable Ponzi schemes from the past ten years, with combined losses for investors of $23 billion. It estimates that up to half of those were affinity-based. No one has a reliable number for smaller frauds over the same period, but guesses range from $5 billion to $20 billion. In all, affinity-fraud losses in America could be as much as $50 billion."
H/T Zvi Bodie