A recent well-publicized Chinese-government study, for example, claims that only half of the Chinese population are capable of speaking Mandarin ("neng2 shuo1 Pu3tong1hua4"; http://www.beijingtimes.com.cn/news.asp?newsid=88058). This is likely to be shocking only to those who never venture outside of the circles of urban, well-educated elite (of whom the Chinese Wikipedia can count many members).
You are confusing Mandarin with Chinese in its written form. Yes, only 50% can speak Mandarin, but over 80% are literate and they can read Chinese characters (although not in Mandarin). Chinese dialects have more or less the same grammar (probably due to the interaction among the people over thousands of years), therefore not being able to "speak" Mandarin does not mean they cannot "read". The fact is, zh: is a *Chinese Wikipedia*, not just a *Mandarin Wikipedia*.
[[User:Formulax]]