I think all users are viewers but not all viewers are users, in the sense that the viewing experience is the classical visual media experience. You can substitute “listeners” if you want to think about it for radio. It's the classical conventional media experience. It's one-way. “Passive” is the wrong word, because anybody who's ever listened to a radio program and really enjoyed it knows they're not being passive, but it's a passive experience in the way it's delivered.
On the other hand, the term “user“ denotes an ability to engage in interactivity. Increasingly we are finding that lots of our viewers are becoming users at the same time. We've got tens of thousands of people watching Hockey Night in Canada on any Saturday night who are online at the same time, engaged in a live conversation with other people who are online about what's going on in the game. Sometimes with the program itself, we actually build that content into the conversation.
So that's the difference.