The end result of community management isn't engagement.
The end result isn't improved satisfaction ratings, better sentiment, greater awareness, or great feedback. The end result (with a few non-profits aside, is greater profits).
This is true of everything an organization does (amateurs/hobbyists can skip this post).
We need to get good at talking about the realized benefit, not the step before the benefits.
For example, increased engagement/sentiment might lead to repeat purchases, improved retention rates, new customer acquisition. If that's the case, then measure repeat purchases, improved retention rates and new customer acquisition amongst the people in your community.
In our survey a few weeks back, many people working on internal communities wanted to help employees share information. That's great, but what's the end result? Is it reduced costs, increased productivity? You can measure this.
Link the thing you're immediately trying to achieve to the end result for the organization.