A report from Chitika, "The Power of Free: OS X Mavericks Users Generate 40% of Mac Web Traffic Five Months Post-Release" says that recent usage data for Mavericks indicates that the free update has accelerated its adoption compared to previous OS X versions.
Almost exactly five months following Mavericks’ release, the operating system’s users are generating 40% of U.S and Canadian Mac OS X-based Web traffic, a figure that dramatically outpaces Mavericks’ predecessor, OS X Mountain Lion. At 40%, the share of OS X Web traffic generated by Mavericks users is approximately six percentage points higher than what OS X Mountain Lion achieved nearly 14 months following its public debut, and 13 percentage points higher what was recorded at the seven-month mark.
This chart from Chitika shows the current distribution of OS X versions, with Mavericks in green at 40 percent of the users.
There is some interesting analysis that explains, for example, the contrast with iOS 7.x adoption that now commands 80 percent of North American Web traffic.
One explanation is that Macs are typically kept for a long time and older models may not be upgrade eligible. Another is that iOS users get an active prompt to upgrade while OS X users do not.
Future releases of OS X should do even better, according to Chitika, as the OS takes a more active role in notifying users of free updates.