Just 7.7% of devs are unpaid.
As its importance has grown, development of Linux has steadily shifted from unpaid volunteers to professional developers. The 25th anniversary version of the Linux Kernel Development Report, released by the Linux Foundation today, notes that «the volume of contributions from unpaid developers has been in slow decline for many years. It was 14.6 percent in the 2012 version of this paper, 13.6 percent in 2013, and 11.8 percent in 2014; over the period covered by this report, it has fallen to 7.7 percent. There are many possible reasons for this decline, but, arguably, the most plausible of those is quite simple: Kernel developers are in short supply, so anybody who demonstrates an ability to get code into the mainline tends not to have trouble finding job offers.»