Want to know how well your software is doing? Look for the absence of complaints. Compliments are always fantastic to receive, but are not consistently delivered, and hence not representative of your software’s ecosystem. Things working great is status quo; it’s expected.
On the other hand, people are negative by nature — Murphy’s Law comes to mind — which means that a good percentage of your users won’t hesitate to complain when things do not go as expected. Other upset users who do not complain will simply find an alternative solution, so a shrinking user base is another good indicator (but a growing user base could be the result of numerous factors).