Well, I can try to recreate my math, since I can't find the post, and it's admittedly imperfect guesstimates.
The Galaxy S2, which is the most popular CM phone has 129,274 recorded installs on a userbase of 20 million+ (
CMStats)
If we take CM installs as a proxy for rooting (which obviously doesn't correspond 1:1, plenty of people stay stock-rooted, but we'll try to compensate for that) and divide 129 274 / 20 000 000 = 0.0064637
So .006% of GS2 owners have installed CM, and reported it. So let's guess reporting rates are only 33% (not sure if that's high or low), so we can multiply everything by three. That gives us .018% of the GS2 userbase. We're still taking about infinitesimal numbers here. Let's say the reporting rate is only 10%. We're still talking about .06% But let's stick with that .018% number.
Let's say that my proxy is really really imperfect, and for every user that installs CM, there are 10 who root but never do. Sticking with the .018% number and multiply that by 10 we get .18% of GS2 users. It's going to take a LOT of multiplying to get us even to 1% of GS2 users. I think no matter how you do the math, the number of users relative to the total userbase is just tiny.
Correct me if I'm wrong though, these are just estimates.