IIRC (and I fear I don't RC), the web industry hated this idea because they couldn't control and monetize the conversation around their content. Of course, we now do this exact thing with sites like Slash Dot, Digg, and Hacker News, only there is this step of going to the social site to see the popular links.
If you go to a page and wonder what HN has to say about it, you have to do a search for the URL by yourself. (Perhaps there is a browser add-on that does this?)
Taking comments to a place where you could annotate the document and not just discuss the page as a whole is the next step for sites like HN. Of course, there is the pesky problem of the sites hating this and using every legal tool in their arsenal to prevent you from presenting this as an interface.