Investors are one thing, customers are quite another.
Many defrauded were Ag commodities producers who had place money with them to backup some of their trade position. They never "invested" in MF Global. Some actually purchased "glold bullion" and it was held at MF Global's facility. They owned it, it is a tangible asset, but from what I have read they are treated just like the other "customers" in the Bankruptcy. They no longer "own" the gold they paid for.
MF Global Theft Leads to CME Breakup?
Fox Business video
http://video.foxbusiness.com/v/1377288026001/gasparino-break-up-of-cme-on-the-table/