Tracking User Engagement
Check your Scorecard Bulletin Email Stats - these are a good sign of user engagement - is the email being opened, is it being clicked on?
Tracking User Behaviour Change
In Reporting you can generate "Shift" Reports. These show the percentage change for each quantile (e.g. quartiles) between two releases. You can also create a user shift report that shows change for each individual user. These reports show quantitatively how behaviour and scores have changed over a period.