As noted previously, our audience is not representative of the web as a whole. We have a large Mac readership, a fairly tech-centric readership and a fairly affluent one. Still, these are big, big changes over a short period of time and sort of eye-popping if you're still used to the idea that Windows is (ok, was) the default operating of the computing world.
Looking more deeply into the numbers, there are two basic trends. One is the rise of Mac market penetration, which is higher for our readership than for the web at large but still growing. But the big factor is mobile.
As you can see, the drop off in Windows usership seems clearly correlated to the growth in Mac usership through 2009. Then Mac levels off and even drops a bit. And the growth is all in mobile. Note that for TPM mobile usership is roughly doubling every 12 months.
The additional wrinkle here is that 77% of that mobile traffic you see in the chart is from iOS devices, i.e., iPhones, iPads, iPods, etc. So give or take, around 40% of the visits to TPM come from computers or devices that use an operating system built by Apple. Compare that to 20% only 5 years ago.