I think the Contributor Summary macro (http://confluence.atlassian.com/display/DOC/Contributors+Summary+Macro) will do what you want:
The above should show a list of all users with the number of comments that they have made and will include users who have not made any comments at all.
If you have a look at the documentation you can tweak the macro paramters to also include the pages/blogpost on which the comments were made, restrict to pages only, etc.
Hope that helps?
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