Magento Version Distribution
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This post is over 8 years old - it might be slightly outdated and consigned to the history books.
Magento Version Distribution - September 2014- Larger Version
Going forward you'll be able to track all versions of Magento and their coverage on a weekly basis, just search for Magento and the version number to find the specific trends report.
You will also be able to download reports on which version of Magento websites are using, letting you find customers that might need help upgrading or get in contact with ones that are working at the cutting edge of eCommerce software.