> Do you consider it credible that they have received, this year so far, somewhere between 5 and 10% of the entire volume of fiat currency that has so far been put into cryptocurrency in its entire history so far ?
Yes I do: if we look at the top exchanges volume wise according to coinmarketcap[1] we can see that 5 out of 10 exchanges only support USDT as anything resembling any type of FIAT, and another one has a direct market for USDT into USD (Kraken).
Number 2 on the list is an exchange (Binance) that was launched less than a year ago and does (according to their own data) 3.5 billion USDT in daily volume and according the tether blockchain owns over 500 million tether (technically their users own them).
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So yes I do believe a ton of money is flowing into the tether ecosystem (mostly to these new exchanges that don't do FIAT like Binance, Bittrex, Poloniex, Huobi, etc).