Latest Steam numbers show Linux, Mac gamers almost equal
The article says there are almost as many Steam Linux users as there are Mac users, after only a few months of availability. You should also consider there are like 5x fewer games on Linux compared to Mac, and I've also seen developers report 3x the revenue they got on sales from Linux compared to Mac. So the impact of Linux users on Steam is much bigger than that of Mac users.
Also, if you're a developer and you're considering Mac already, there's no reason why you shouldn't support Linux as well at this point (looking at you Blizzard!). Linux users probably have more powerful hardware, too, on average, as they can just install Linux on their PC's, while most Mac users probably have a Macboo Air with integrated graphics, or some old Macbook Pro.