Do many people use these features? Most of the people that I know who use Google Reader, myself included, just use it as an RSS reader. Sending it to other social networks is the one thing that I do use, and they won't be changing that.
I would prefer an integration of G+ circles with sharing preferences on Reader, but I don't want to give up the Reader interface for the much less utilitarian G+ interface. I'll wait and see what this change brings.
I am a heavy user of the information on how many users are subscribed to a particular feed, which could be considered a social feature. (The information is behind the "show details" link in the "view onto" a feed.)
also, I am subscribed to one other user's "shared link blog" (a view assembled by Reader of that user's "shared items"), which the OP says is definitely going away, and three users are subscribed to my shared link blog.
The "shared link blog" that I am subscribed to is at http://www.google.com/reader/shared/patrissimo although admittedly it is no longer active as the writer (actually, "editor" or "sharer" is more accurate) has moved to using Twitter for sharing links (which I do not like as much).
http://www.google.com/reader/shared/patrissimo
The difference between an ordinary blog and a shared link blog is that in a shared link blog, the posts (I want to call them "items" but know of no one else who calls them that, so "posts" it is) are written by people other than the owner (i.e., "patrissimo") of the shared link blog. All "patrissimo" did to put the post in the shared link blog was to check the box labelled "Share" or the box labelled "Share with note" at the bottom of every post in Reader (where the box labelled "Mark as read" is also to be found).
Well, more precisely, if he checked "Share with note", he also had to type in text of the note to be attached to the post. And there is a bookmarklet that he could have used instead of checking one of the two aforementioned boxes.
My question, then, was whether or not we'll still be able to share within Reader and see items shared by other people. Based on what I've read on this post and elsewhere, it seems that the answer to my question is no - instead that functionality is being moved to G+. Which seems to me to be a rather strange interaction.
Note that making the information public might not be the current default behavior of shared link blogs, but if it is not the current default behavior, then for those users like me and "patrissimo" who have opted to make the information public, the information is (as far as I know -- I guess it could be disallowed by Google Reader's terms of service) available to all services on a non-discriminatory basis.
If you read a lot of feeds (According to Google, I have 347 of them), it makes it that much easier.