Before Amazon Linux, Amazon used RHEL 5 and RHEL 3 internally on both bare metal and with a customized Xen hypervisor to run virtual machines.
RHEL has been the default enterprise Linux distro for many years with SUSE as a second choice. Before that, most enterprises that were not running Windows were running Solaris.
Ok, but still this doesn't indicate any real popularity: a small handful of decision-makers picked RedHat as their standard OS for probably tens to hundreds of thousands of users, plus a ton of servers.
