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The blockchain is really transparent. You can see customer funds @bittrex and @poloniex. It isn't being powered up.

Exchanges do get to keep the interest generated on customers' SBD funds , unless they decide to credit it to customer accounts. None has done that afaik.

No exchanges offer lending of liquid STEEM for shorting yet but I'm sure that will come in time.

@jl777 see my EDIT in my post. Neither have powered down. I was looking at their Steem holding (representing client funds) not their MV representing Steem Power where Bittrex and Poloniex are nowhere to be seen...