It happened back when "Social" was a big thing and Google was trying to push everyone onto its erstwhile Facebook competitor Google+ and it's erstwhile Twitter competitor Google Buzz.
First they killed the social feed and made the button auto-post to your Google Buzz account, but Buzz being a version of Twitter, had a lot more noise to signal so nobody used it that way.
They probably eventually killed it because they preferred having their development resources put towards more exotic social media efforts than maintaining a thing that didn't really make any money.
This long article has more of the sordid details if you're interested: https://www.buzzfeed.com/robf4/googles-lost-social-network?u...
All thanks to an ex-MS "cookie licker" (the term was used by Google people after he left the company).
There are clients that kind of did, but one of the cool things about Google Reader that a lot of people miss is the social feed. You could share articles you found interesting and your shares would show up on the feeds of anyone who followed you as just another RSS source. It's just an extra folder of articles from your GChat friends that you can read at your leisure.
And it's people selecting articles from their normal reading rotation, so it was refreshingly clear of clickbait. People usually shared stuff because they thought their friends would find it interesting rather than distributing content designed to "go viral."
Once the ecosystem fragmented nobody has the critical mass to replicate the social features. It was nicer than what Twitter or Facebook do because it wasn't in your face.