From the very beginning of Silverlight penetration has been an issue. As a professional developer living of Silverlight and as a Silverlight MVP, I often face questions about the Silverlight penetration from customers.
Some facts: The Flash plug-in is installed on almost 97 percent of all computers whereas the Silverlight plug-in is installed on about 70 percent of all computers. http://riastats.com/
Silverlight is primarily used to create enterprise components, tools and sites. If you take a look at the tools used to create Flash versus Silverlight (Visual Studio + Expression Studio) there is no doubt Silverlight are miles ahead of Flash. If you look at the advantages of being able to reuse existing resources and knowledge (.NET and Visual Studio) from an enterprise perspective Silverlight will beat Flash any day of the week.
In my opinion, the penetration of Silverlight has now reached a level where it’s no longer a barrier for enterprises to use Silverlight – even on public faced websites. The many advances of Silverlight will overshadow the lower penetration compared to Flash.