WebGL Stats
webglstats.com
webglstats.com
As for iOS, I wasn't aware of the Safari mobile versus iAds thing. That seems like an odd tech decision, but I guess it makes sense.
(That said, I'm one of the few people lucky enough to have Intel graphics hardware, so I have pretty great Linux drivers. I might feel differently if I were forced to use something from nVidia or ATi.)
[1] http://news.softpedia.com/news/Internet-Explorer-Won-t-Be-Su...
[2] http://games.greggman.com/game/webgl-security-and-microsoft-...
This is probably not the case of the majority but I think that people with this kind of problem are still numerous.