At the end of 2013/beginning of 2014, a survey was conducted to see how much 3.x is catching on.
The survey itself is https://www.surveymonkey.com/s/N5N5PG2. Note that recent takers of the survey will not see their results in the PDF below.
The survey was publicized via posts to comp.lang.python, python-dev and hacker news.
You can find the results here in PDF format: 2013-2014 Python 2.x-3.x survey.pdf.