[ojbyrne]

This seems to be the authoritative source for that number: http://w3techs.com/

and the methodology:

http://w3techs.com/technologies

"We investigate technologies of websites, not of individual web pages. If we find a technology on any of the pages, it is considered to be used by the website. We include only the top 10 million websites (top 1 million before June 2013) in the statistics in order to limit the impact of domain spammers. We use website popularity rankings provided by Alexa (an Amazon.com company) using a 3 months average ranking. Alexa rankings are sometimes considered inaccurate for measuring website traffic, but we find that they serve our purpose of providing a representative sample of established sites very well."

I would argue that the numbers should be weighted by traffic - weighting all of the top ten million websites equally doesn't give an accurate picture.