Exchanges total client assets?

[npiguet]

Hello,

While it is fairly easy to guess how much revenues BTC exchanges get from trading (just take the daily volume and multiply it by double the transaction fee), I can't figure out a way to know how much assets their clients have deposited into the exchange.

Does anyone know where I could find that number for the major exchanges (btc-e, bitstamp, mtgox (before shutdown))?
Has anyone heard any rumors regarding the total deposits in one these exchanges? I haven't been able to find anything.

Thanks for your help!