According to Netctaft's Web Server Survey, the percentage of websites using Microsoft's IIS is rapidly decreasing. Netcraft's survey covers a total of more than 340 million websites.

The percentage of websites using Microsoft IIS has decreased to such an extent that it is now at the level that it was before 1998. Within a one month period between May 2011 and June 2011, Microsoft IIS lost as many as 1.4 million host names while Apache gained 21 million host names.

With the market share of Microsoft IIS down to around 16% only, Apache with a market share of around 65% is the only major web server software left. Other software like nginx also saw a decrease in the market share in the May-June 2011 period - but not as much as that of Microsoft IIS.

Web Server Market Share

The graph above, however, suffers from a very critical flaw. It includes a number of parked domains. This means that it can be easily influenced by domain registrars switching from one software to another. A more realistic representation will be given taking into account only the active domains. Netcraft has also published such a graph taking into account only the top one million domains.

Web Server Market Share For The Top One Million Websites

This graph, however, tells a somewhat different story. It too confirms that Microsoft IIS is on the decline, but provides a more realistic rate of decline. While the market share of Apache has remained largely constant throughout, it is interesting to note that the decrease in Microsoft IIS's share is being taken up by the increase in market share of nginx, which has been steadily increasing throughout from arounf 1% to 7% in about 30 months.

You can see Netcraft's report here.