I am building a consumer social app. After running a couple of NPS surveys, we're sitting at around ~70. That being said, our week 0 vs week 4 user engagement % (defined as % of users who performed an "engagement" activity) is dropping from 50% to high 15%.

The app has only launched for 2 months, the first of which was more of a beta period (and which I don't have analytics data for), so I can't quite come up with month-by-month metrics.

What exactly does this mean when user engagement is on the downtrend, but NPS is relatively high? Does this mean there are a small % of diehard users, but not enough to keep most of the users engaged? Are the engagement metrics meaningful on a week by week basis even? I'm not sure how exactly to interpret these metrics and if I'm on the right path (there's no clear trend in user engagement by cohorts).