Certainly Riastats.com is one source. (for silverlight and Flash penetration) and for Linux usage
http://marketshare.hitslink.com/operating-system-market-share.aspx?qprid=8
http://gs.statcounter.com/#os-ww-monthly-200906-201005-bar
and here
http://w3counter.com/globalstats.php
Silverlight is not going anywhere and neither is its support. Moonlight has lagged behind but its not as if there is no attempt at getting it up to speed so silverlight is not strictly closed. The OP to my understanding is focused on games with a tie in to moodle. I am curious though . What open source platform presently creates modern games in any reasonable productive environment since people here look down on flash and silverlight. I mean the compliance argument is all well and good but its just entirely overboard to me to now claim that it addresses every programming need. I will openly admit that i have no plans to move over all my game programming complete with logic to HTML5. Anyway heres a balanced article on the subject for those sucking the Kool aid a bit too hard.
http://gizmodo.com/5461711/giz-explains-why-html5-isnt-going-to-save-the-internet
And yes. As long as Flash is around the same arguments can be made for silverlight. Why Am I into this discussion? because I am in a simliar position to the OP toying with the idea of using the tremendous opportunities to learn that games have and in tying it to a system such as Moodle to create ar richerr and more enjoyable learning experience. Haven't fully decided whether to do that in Silverlight or Flash but having worked with both realize that Silverlight IS a solid alternative to Flash . Also must admit nothing quite compares to visual studio but thats a less important point.