From GoSquared’s real-time OS X version distribution page:
The graph shows the percentage of visits tracked by GoSquared on over 50,000 sites.
Just over a week since its launch, Yosemite is already installed on 1 in 5 Macs. Interestingly, it looks like most of the early upgrades are coming from users of Mavericks, which was the current version of the OS prior to Yosemite. The share for older versions (Mountain Lion and Lion) has remained essentially constant since Yosemite was released, which was the same phenomenon I pointed out last year.