A recent report from eMarketer suggests that number is about 33 Million
Personally, I would however recommend you to look at other sources too. eMarketer is usually a reliable source, but I am not fully sure of this study, especially because Twitter is banned in China, and the study lists China as the leading country in number of Active Twitter users. Some users may be accessing using proxy-services, but still this does not explain the extremely high usage.
See question on Quora