Netcraft has been tracking the number of web sites on the internet for many years. Every month, they publish their latest numbers. As of May 2012, their chart shows close to 700 million sites (the blue line is the number of unique domains, the red one is sites with unique HTML which excludes placeholder templates like landing pages for newly purchased domains):

As you can see, the growth is impressive and unimpeded. Also:

• The total number of web sites seems to follow Moore’s Law and double every 18-24 months.
• At the current rate, we will hit 1 billion sites in 2013 and 2 billion sites in 2015.
• Over the years, the number of web sites seems to be roughly equal to the number of people on the internet.
• If WordPress continues on its current trajectory, there will be 300-500 million WordPress sites by 2015.