I see them more as "victims" of it, who are subjected to it without giving much consent at all. Somebody loads a blog article or some other web page that uses Disqus for accepting and showing comments, and without necessarily wanting to use it the page's visitors have become "users" of Disqus.
The site owner chooses a commenting system. People who want to comment, comment using it. Victims?!
Reading your comment again, I wonder what the true definition is. Is the number based on page views? Or is it the number of people who actively interact with the widget (say, posting a comment) after it has loaded? Sounds like you're assuming the later metric while the 'victims' comment is assuming the former.
Edit: thinking about it, 1 billion unique people, that sounds like the victim class to me.
Heck, Disqus is better than 90% of the forums out there (Hi, there, PHPBB or vBulletin!).