Jeff Kunins, the Group Program Manager for social networking across Windows Live, wrote an interesting post about the history of Windows Live Messenger on the Inside Windows Live blog. 

He tells us that the instant messaging category got going around 1996 with the debut of ICQ. Over the next two years, each of what are now the leading IM services launched in rapid succession: AIM, Yahoo! Messenger, QQ, and MSN Messenger. In the six years following that, instant messaging grew fast to over half a billion users sharing hundreds of billions of messages every month!  

Which brings us to some interesting stats about Windows Live Messenger: 

• More than 300 million people in 76 countries and 48 languages use Messenger every month  
• Messenger users now represent:
  - 65% of all Internet users in Brazil
  - 48% of all Internet users in Canada
  - 48% of all Internet users in Spain
  - 47% of all Internet users in France
  - 40% of all Internet users in Italy
  - 39% of all Internet users in UK
• People use Messenger for 163 billion minutes every month, which is about 9.4% of all time consumers spend on the Internet worldwide.
• More than 40% of the users sign in each day (more than 130 million daily users)
• Every day, those users share over 1.5 billion conversations and send more than 9 billion messages. At peak times, that drives more than 40 million “simultaneous online connections,” (the number of people signed in at the same time).

More fun stats can be found in the blog post. Being responsible for partnering with and connecting to social networks and other web services, he further explains that IM services really were the original "social networks."