That sharp drop is a reduction from 29,049,223 (May) to 21,898,527 (June) active sites. Netcraft explains the drop like this:

A reduction in activity at Microsoft Live Spaces was responsible for the large drop in the number of Microsoft-IIS sites detected.

This makes me wonder exactly how many of those IIS-hosted active sites are actually run by Microsoft (or its partners). The fact that just one of Microsoft’s services was responsible for over 7 million “active sites” – or 25% of the total number of active sites detected as running IIS in May 2009 – makes me wonder how valid the IIS numbers are in the webserver survey. I think this suggests IIS use is far less prominent outside the Microsoft campus than the ‘active sites’ numbers indicate.