"Technorati is now tracking over 7.8 million weblogs, and 937 million links. That's just about double the number of weblogs tracked in October 2004. In fact, the blogosphere is doubling in size about once every 5 months. It has already done so at this pace four times, which means that in the last 20 months, the blogosphere has increased in size by over 16 times."
John Hawkins goes deeper onto some fairly high numbers of actual blogosphere members.