"...there had been 350 million installs of Silverlight since 2007..."
Largely because Silverlight appeared as either a recommended or critical update to Windows in Windows Update. So unless you were monitoring updates and applying them yourself; it would have been automatically installed as a "recommended" update.
One question though: if SL3 will allow the download of library code, what's to stop malicious code being injected into it? I've not looked at it at all, but does SL have any kind of security context? What can SL do to your machine? Is it as leaky as ActiveX or did MS actually think about security with it?