Enfodesk's & International Analysis' soon to be released 'Q1 2009 Chinese Instant Messaging Market Report' has some interesting numbers:

• 1.6 billion registered IM accounts in China (+4.1% compared to Q4 2008, +31 % compared to Q1 2008)
• Number of active accounts (accessed more than once a month): 510 million
• Constant growth rate of about 50 million new Users/quarter