MT Gox was openly allowing people to trade an item that did not exist. The were obviously paying withdrawals from fresh deposits since they were missing more than 700 000 coins that were still being traded. That is a ponzi my friend.
As long as there were both a buyer and a seller on each side of the trade, and Gox was only a middleman collecting fees, then no... it wasn't a ponzi.
Bad accounting, and/or having your assets stolen through a fraud conducted by unrelated third parties does not make your business a ponzi.