Introducing the WordPress Download Counter chart. Historical chart displaying the total amount of downloads for each WordPress branch, including both major and minor versions. Source
The chart is based on the data from the official Download Counter, segmented by WordPress version.
The WordPress download counter was added after the release of WordPress 1.0. It simply counts the number of downloads and it was meant to evaluate the number of sites using WordPress.
But the counter was not a reliable source to evaluate the number of active WordPress installs because when a new minor version is released, it triggers the auto-update mechanism that downloads new versions on millions of sites.
With the introduction of automatic background updates in WordPress 3.7 we have seen a rise in Minor WordPress Versions. But from the chart I can conclude that the 3.7 auto-updater is not working as expected as the total downloads count in very low.
Also, all versions since 3.8 have a very strong match when comparing the Version Lifespan with the version Downloads Count. There is a strong correlation between the lifespan and the downloads. Versions with longer lifespan have more downloads.