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The top most exchange binance announces it will provide double referral commission to the extent of 40% for all its account holders that hold five hundred binance coins or more, this referral starting from 19th of may 2018 0AM UTC.
The users not under this criteria will get 20% reward as usual.
https://support.binance.com/hc/en-us/articles/360004047051-Binance-Doubles-Referral-Rewards