Ask HN: Browser Plug-in Adoption Percentage?

A friend of mine asked me whether I knew of any stats detailing the average percentage of users that will download a plug-in to view website content. I couldn't find anything, so I thought I would ask HN. Is there any study, or evidence out there that details these percentages? For instance, when faced with the prospect of downloading a plug-in such as Adobe ConnectPro to view website content, how many visitors on average would download it, and how many would pass?