Alibaba Group today announced the long-rumored investment in Sina Weibo, the microblogging service under Sina Corp., taking an 18% stake for $586 million. Hence Sina Weibo is valued at approximately $3.26 billion — the market cap of Sina as a whole, including an online news service and Weibo, was about $3.33 billion before the announcement. Alibaba Group is also granted the option to increase its stake in Sina Weibo to 30% at an unspecified time in the future. One week ago, Sina Weibo announced partnership with Alibaba’s Taobao to feature contextual e-commerce ads on Weibo. Also Sina Weibo and Alibaba’s e-commerce sites are complementary: it was found that 3% of users would visit Sina Weibo immediately after visiting Taobao and 3.2% of users do the exact opposite (in Chinese).