You might not have heard of Mixi, but it’s big in Japan, one of the largest social networks there, in fact. Now it’s ready to spread its wings and head intoChina, with plans to set up a Shanghai subsidiary this spring, according to Thomson Financial. Mixi was founded in 2004 and has becomea major force in social networking among the young in Japan, and claimed to have over 13 million members, up five million from a year ago, even though MySpacelaunched in Japan in 2006. Mixi will be going head-to-head against MySpace in China, too, since the Rupert Murdoch-owned company launched its own service there last year. But the Japanesedon’t seem too perturbed. "We see China as a market that has huge potential for the Internet, including in mobile wireless Internet services," Mixi said in a statement."In terms of the future growth of our business, overseas expansion provides an important opportunity to expand revenue,"