Here's my attempt
Assuming its a 20% increase. Can I ask what the feature does? - Some random answer
Is it increase YoY / WoW/ or increase in a day?
Is the increase for a specific region or demography?
Is the increase seen on app or web?
Let's say the increase is seen WoW for all region and demography for that feature
Engagement of feature X increase by 20% whereas rest of the FB feature is stable. End result overall increase by 20%
Engagement of feature X increase by 20% whereas one of the other feature of FB has decreased by 40%. End result overall decrease.
Engagement of feature X increase by 20% and one of the other features decreased by 20%. End result overall stable.
In order to know if Case1 is true, I would collect the following metrics
* # of DAU and MAU: if the overall engagement has increased, we should see an increase in DAUs
* Avg time spend by a user should be higher if the overall engagement has increased.
* Revenue by ads. This should also go up
* Engagement on FB is tied to posts and reactions. We can check if the # of posts or reactions have gone up.