Red Hat's Fedora Linux 10 has been out since the end of November 2008, and is now hovering around the 1 million installations mark. Fedora uses a system to measure active installations that check the update repositories in order to determine how many installations are in use.
When combined with other actively used Fedora distributions as of Feb 16, 2009, Fedora's counting method reports 12,188,598 Fedora Linux installations across Fedora 7, 8, 9 and 10 releases. Obviously, some of those have been updated to later version already, and might have been counted twice.
On Fedora 10 in particular, in contrast with adoption for Fedora 9 for a similar period, Fedora 10 is at 115 percent adoption (that is a greater adoption rate for the first 12 weeks of release for Fedora 10 than Fedora 9).