We have an online application. Lots of people are using it but of course not everyone. My goal is understanding the recently inactive people and what I can do to make them more active. What I'm doing:
What else should I be doing?
The "champions" program is awesome, but will not help you with this particular problem because none of them fall into this category.
Survey is a good idea. It will probably generate more questions than it solves, but they'll be the right questions. For example, if an answer is that they're using a competitor, you need to know why that is, and that will drive next steps.
On metrics: Which features did they use and not use? It's possible that people are leaving because they just don't know how to best use the service. Here a way to help is through education, tutorials, or follow-up emails.
Another thing to try: Start a monthly newsletter. Highlight success stories, cool features "you might not know existed," or how to solve problems that you might not know you could solve. This gets inactives thinking about you again, and possibly educating them about how to better use your tool, which in turn might make them successful.
(Newsletters are useful for other things anyway, like announcing new versions, promoting external bloggers in exchange for reviews, highlighting customers, promoting your own blog, running a survey less intrusive than actually sending out a bunch of emails, etc.)