Facebook in Chinese (Simplified, I presume) is available. The Wall Street Journal (a fine publication I read every morning in my local store until the Polish guy comes over and tells me to either buy it or get out) has the story.
The Chinese site can be accessed by going directly to zh-cn.facebook.com. However, this takes me to an English logon screen (presumably because of my Irish IP address), and then logging in with my usual details makes me think this Chinese version is rendered from a single global instance that’s just been cross-mounted with a .CN domain.
However, it does not – unlike other “international versions” – allow for “social translation” (see the “Translations” options in the following screen shot):
You can get the Chinese UI by simply switching your language option. Go to account > Language.