At the end of 2013/beginning of 2014, a survey was conducted to see how much 3.x is catching on.
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.
Results are also available as a single chart. Unfortunately, you need to mouse over the labels to see the full question text.