It's more analogous to counting people as drivers of cars when in reality they're only ever passengers.
Nobody's saying that Linux servers aren't used. What the GP is saying is that you can't count *every single user* of some popular site as a user of the OS that site runs on.
Or to put it another way: Let's say 70% of the Web-browsing public uses GMail. (which, of course, is a number I pulled straight out of my ass.) Does that mean 70% of the Web-browsing public are Linux/GoogleOS/whatever-OS-GMail-runs-on users? No, and to try to say otherwise is just outright skewing the numbers. They're GMail users, and that's all you can say about them. It makes no sense in this case to say that Linux use is up from a user perspective.
Now, had you framed it in the context of the servers themselves - with more users of the service equating to needing more Linux servers to cope with the load - then you might have a point. (though even then, it's still only use by one company.)