According to Matthias Gelbmann of W3Techs, 25% of the sites it surveys are using WordPress. The milestone comes two years after reaching the 20% mark.
http://wptavern.com/a-quarter-of-the-top-10-million-sites-ranked-by-alexa-use-wordpress
A Quarter of the Top 10 Million Sites Ranked by Alexa Use WordPress
Jason and Company Notes 