Taken from Techcrunch

The user numbers coming out on Google Talk are staggeringly terrible. Comscore usage numbers show that nearly a year after launch Google is a distant, distant 4th after MSN, Yahoo and AIM. They hold a pitiful 1% of total instant messaging market share, with 3.4 million unique users in May 2006

I will say Google has some pretty smart and useful features like integrating a web version of GTalk in GMail itself which is very useful in places where IMs are blocked/banned or where you cannot install softwares. Microsoft and Yahoo! should learn something from this and integrate more features directly into the inbox.