Qihoo 360

Zhou founded 3721 in 1998, a search engine that sold Chinese-language keywords for Roman-alphabet domain names, an early and distinctly China-specific internet business model.

China's operations, but differences between Zhou and Yahoo's management led Yahoo to sell its China operations to Alibaba in 2005, after which Zhou departed to start Qihoo 360.

As of November 2018, Forbes ranked Zhou #45 on its China Rich List and #135 on its global Billionaires list, with an estimated net worth of $4.7 billion.

Left Yahoo! China after management disagreements and founded Qihoo 360 following Yahoo's 2005 sale of its China operations to Alibaba.

Qihoo 360 grew into a major Chinese internet-security company, with Zhou continuing as co-founder, chairman and CEO.