Its not new companies merge in each other or a big company breaks down in further companies. Sometimes companies merge in the big profitable companies or sometimes small parts separate from big companies and make their own identity. Poloniex separated themselves from the Circle company and started own services. But these services will not be available for U.S Customers at the moment.
According to Circle Blog
“As detailed in a separate announcement from the Poloniex team, US customers will no longer be able to trade on the exchange starting on November 1, 2019. US customers will continue to be able to access and use their wallets and withdraw funds through wallet and custody services operated by Circle until at least December 15, 2019.”
More said in the Blog
“It is bittersweet for Circle to see this incredible product and business spin out on its own … We’ve made enormous progress with Poloniex, including massive infrastructure improvements, adding more fiat options with USDC integration, launching best in class native apps for traders, and building global operations capabilities that can deliver excellent customer service.”