In their own words, “the Yahoo! Social Bar allows you to effortlessly share your online reading and viewing activity with your friends, and to easily discover theirs.”
And that’s exactly what is does as you can see from the snapshots here.
Yahoo’s social news integration lets their users know what their Facebook friends have interacted with on Yahoo.com & offers auto-scrobbling of what a user has read to their Facebook Timeline.
Despite being an “always-on” seamless integration with Facebook, Yahoo’s implementation feels like it cares about privacy because you have the option to let it ask you if you want to share every single time or turn “Social” off entirely to read privately.
This was released back in 2011 & it hasn’t changed much.See a news article from that year here & Yahoo has background info on this implementation here.