Hi Mickey

Its a good question. I think discussion is actually implicit in the diagram.
The overall discussion is the set of all other communications that cite some
portion of the original. You can then imagine reducing that large set by
specifying all communications that come from some subset of sources
(friendfeed and twitter, or formally published reviews), or that cite just
the conclusion (or just the data, or...). By following the citation trail
you can also actually get a threaded discussion (or a webbed one if there
are cross citations).

An editor might aggregate some subset of these together to create a new
communication which would be a curated discussion based on some criteria as
well. But all in all this kind of view does some nice things like explicitly
connect formally published and informally published objects together.