Yes it has. Remember that in the first few years of Job's return, Mac sales were in the doldrums. It's quite likely that the amount of active Mac users didn't really change that much... while the amount of PC users took off. It's only in the last four years that the Mac has started to reclaim some of it's lost market share."
Whatever the last 4 years is called, it wasn't real growth, at best just catching up to losses to hold the 25m number consistent over the long term.
Either Apple has some new cutomers or the "existing base" appear to be buying three times as many Macs as before."
As long as the 25 million number is holding, increased Mac sales can only mean new customers are replacing defections and existing users and buying more frequently.