Weibo (微博) is the name for microblogging (tweeting) in China. While it is extremely difficult to find reliable data regarding Chinese weibo users, official reports estimate that there were 250M weibo users in China at the end of 2011. Comparing this to the 100M total users Twitter reported in September, 2011 puts this massive number into perspective. And from personal experience I can tell you that Chinese microblogging is at least as popular as Twitter in the US and, from what I can tell, that popularity isn’t waning.
The major players — Sina and Tencent
There are two major microblogging services in China: Sina and Tencent. Both of these companies are major players on the internet scene in China, and although Sina was the first of these two to launch a weibo service, Tencent has been catching up quickly and may have even passed Sina in the total number of users in 2011. However, the total number of users of each service should be questioned as there are a lot of “zombie fans”, which are accounts people make up to increase their followers (a.k.a. fans). It is very difficult to get estimates of the number of “zombie fans” out there, but, after March 16th, 2012, changes will be implemented to try to ensure that accounts are tied to real people.  Anecdotal evidence suggests that there are less “zombie fans” on Sina than there are on Tencent, though, again, with the changes expected to be implemented this year to control the “zombies” this might all get sorted out very soon. Although there are other weibo services, Sina and Tencent are by far the most popular and, at least as of this writing, no serious competitors are threatening their dominant positions.
So what about Twitter?
Twitter is blocked in China, so while it is used by some dissidents and makes the Chinese government rather upset, it’s not currently available to the average Chinese citizen who doesn’t have access to a VPN. The likely reason Twitter is blocked within China is due to Chinese censorship requirements, though with the recent addition of Twitter’s ability to censor by country it may not be long before Twitter enters China to threaten the dominant positions enjoyed by Sina and Tencent.
 See http://online.wsj.com/article/SB10001424052970204643804577101522579231922.html for more on the planned restrictions and possible reasons tied to controlling dissent
 See http://www.guardian.co.uk/world/2011/oct/26/china-social-media-censorship for more on increasing censorship of social media