From the article:

After all, before Google Reader’s sharing features were converted to corresponding Google+ ones, users of the soon-to-be-killed service used to share like crazy. Shih postulates, with plenty of logic, that this activity dropped as Google pushed Reader users to sharing on Google+ instead.

That is in reference to the changing from internal sharing to Google+ sharing in October 2011, not shutting down Google Reader. I understand that Google prefers their users to use Google+ over Reader, but I still don't see how shutting down Reader = more Google+ users, even from Google's perspective. If anything, they are removing a service that could have driven Google+ sharing.