There are no official stats. Sometimes cloud providers comment on the popularity of frameworks.
"Appium has been the number one smoking hot framework during the past quarters and there are no signs of this trend cooling down anytime soon. We’ve been providing Appium/Selenium support for Testdroid Cloud now for more than a year and significant portion of Testdroid Cloud test runs are Appium/Selenium runs nowadays."
http://testdroid.com/news/a-new-way-to-use-appiumselenium-with-real-devices-on-cloud
In terms of what "smoking hot framework" means in terms of numbers, I don't know.
