Relevant excerpt from the report:

" Trading platforms that engage in proprietary trading on their own venues uniformly told the OAG that their trading desks had no informational or other trading advantage over customers. The OAG found that significant variation exists in the amount of trading activity attributable to those platform operators. Circle reported that it accounted for less than one percent of the executed volume on its platform Poloniex during the most recent time period reviewed. BitFlyer USA indicated that its own activity accounted for approximately ten percent of the executed volume on its platform. Another, Coinbase, disclosed that almost twenty percent of executed volume on its platform was attributable to its own trading. "