At the high, the marketcap of Clam was about 8585 BTC, and at the May 25 low, the marketcap fell to about 1939 BTC, or a difference of about 6645 BTC. Poloniex allows for 2.5x margin trading, which means the value of the margin longs, plus the value of the collateral was just under 35% of all Clam outstanding. This is assuming all margin long positions were opened at the high and sold at the low, that margin was maxed out on all positions, and all positions were secured entirely by Clam.

The losses exceed the trading volume of Clam during the flash crash, so it is probably safe to assume not all positions were sold immidiately. If no Clams were sold, and their value was written down to zero, as few as 21% of Clams outsanding could have been in open positions. I would believe that Poloniex sold the open Clams positions they were unable to sell in the open market, probably at a fairly large discount.

Poloniex is the only major exchange Clam trades on, so any major price action would have occured on Poloniex. Overall, the losses are the result of poor risk management on the part of Poloniex. They should not have allowed such a large percentage of coins outstanding to be bought in margin long positions. I also suspect they initially were automatically closing out margin positions in mass before someone pulled the plug to automatic licquidations that probably caused a large portion of the price decline.

I don't think this would have happened on a more professional exchange.