Twitter: Fanfou (饭否)
Facebook: KaiXin (开心)
For every popular social networking site in the US, several Chinese copycats have been created--the idea was adopted, but the business model and user experience are often totally different.
Keep in mind that Chinese people are quite open and willing to chat/comment in online forums or using SMS. According to data published by China Mobile and China Unicom, every Chinese cell phone user sent more than 850 text messages on average in 2008. The need for communication with close friends, with people in the same industry, and with whoever wants to chat is evident. Those copycats (山寨) above were created by those entrepreneurs who see the huge opportunity of capitalizing on the localized Chinese social networking service market. Will they succeed? Hard to say, depending on how good the localization is done, how to build a hype(?) for the site, and perhaps how to convince a Internet giant to acquire it......
On the other hand, there are very cool home-grown Chinese online services and also financially successful. Some examples are ppstream, xunlei, and of course, QQ. Their US competitors (if any) are nowhere close to them by all means.