First let me say I have never done any SEO for non-English.
However, I have a lot of Chinese friends and have visited a number of times so the following may be of use.
As well as English & Mandarin you will probably need to do Cantonese as well, this is the main language used in Hong Kong and parts of S.E. China
You may want to SEO using traditional chinese characters (for Cantonese & Taiwan), simplified chinese (for Mandarin) and Pinyin (A Romanized version of Chinese characters used in teaching Mandarin and needed to input chinese characters with a Qwerty keyboard)
Baidu is a must, China's biggest search engine. Although Google is not there at the moment they may return in the future, also (despite what people say) you can still search in China with Google if you are a visitor with a smartphone, I've done this many times.
Social networking-wise, try QQ, Renren, Xiaonei.com, Kaixin001.com, 51.com and Wiebo
Best wishes, sounds like you have your work cut out