From Piper Jaffray's Gene Munster, in his May 12th note on Weibo ("overweight," $25 price target):

“Currently, Chinese Internet users cannot access Facebook or Twitter in China. We note that neither Facebook nor Twitter have been available in China for the life of Weibo, and we believe this market opportunity is a core reason for the popularity of Weibo. While there have been media reports about Facebook’s interest to enter China (less so for Twitter), we believe that there may be some core philosophical differences between how the Chinese government would like social media to function and the overall openness of either Facebook or Twitter. As a result, we believe it is unlikely that either Facebook or Twitter become available in China in the next two to three years.”

Or in the next twenty or thirty years. (If anyone has the Goldman Sachs note on Weibo, I'd appreciate it if you could email it to me.)