Courtney Stanton brought this up around the launch of social. When someone shares a story without typing remarks, there’s no way to reply directly to them. You can reshare the story with your own remarks, but that’s not the same idea.
As Courtney predicted, what you see people do to work around this limitation is put junk characters in the comment box of every share in order to invite replies. This practice in turn makes the share vs. comment counts in the top bar meaningless.
A better fix would be to have a reply button underneath every post in someone’s blurblog, even if they didn’t type anything. The system could then create a blank root comment for that share, under which the new reply thread would go.