Poloniex was a US-based crypto exchange founded in 2014, which in 2018 was purchased by Circle, who intended to get rid of the illegal activity for which it was known. However, when they discovered that the customers who used Poloniex no longer wanted to use it once they were subjected to scrutiny, they sold the platform to Justin Sun in late 2019, who relocated it to the Seychelles and shut down US operations. It appears that the OFAC fine will apply to the US entity most recently controlled by Circle, and not to Justin Sun's operation.
In August 2021, Poloniex also paid more than $10.3 million to settle allegations from the U.S. Securities and Exchange Commission that it had operated as an unlicensed exchange.