Diigo makes any document on the internet a social object by allowing any Diigo account holder with the correct privileges to annotate and comment on the content. For the following demonstrations, you'll need to be logged into Diigo, then, if I've set the permissions right, you should be able to see the examples.
I'm still finding Diigo frustrating and complicated to use - the interface is awful - and I'm not convinced this is the best way to approach the issue.
I much prefer inline comments, so I knocked up a quick demo wiki. This is only a short text, for longer works it would be necessary to balance the tension between continuity of the text and breaking it down so that tagging becomes useful.
To me, the best solution at the moment is looking like CommentPress, an open source theme for the WordPress blogging engine that allows readers to comment paragraph by paragraph in the margins of a text. With CommentPress you can annotate and debate at paragraph level, turning a document into a conversation, as in this example. The comment interface is very familiar and should encourage good participation.
Problem: The Commentpress theme is not available at wordpress.com, so it would mean a self-hosted installation (which would then be able to serve more than one text). Maybe something after the design of StatsBytes would be an acceptable compromise?