Tracking Player Engagement
Check your Release Email Stats - these are a good sign of player engagement - is the email being opened, is it being clicked on?
Tracking Player 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 player shift report that shows change for each individual player. These reports show quantitatively how behaviour and scores have changed over a period.