We all have to fight off the Fog of War. In that light, I would like to suggest temporary across-the-board withdrawal fee increases to cover losses until the deficit is satisfied. This way, there wont be a big run on BTC there, and converting to other coinage bears the same fee. Keep calm and choo-choo on. These are growing pains for a great exchange.
Though this may require some work, I think having tradable fees/shares of poloniex would be preferable, the shares would give dividends (like cryptsy,
mcxnow, etc.), and the selling of those shares would help cover the deficit.
If only 12.3 % were stolen, I'd rather have him not say anything for now and not cause everybody to try to withdraw.
You mean he should Gox his users? Seriously...