To me HN is just a ripoff of Reddit.
1 - Usage is defined by how often an app accesses user data using a token. This does not necessarily mean the user used the site, at least not in the last 60 days (user tokens need to be renewed every 60 days).
2 - Facebook often lags a few days when reporting daily user activity and is somehow inconsistent of when the data is reported. (their open graph reporting seems to be more up to date/realtime )
When that happens appData seems to sometime report the same data point for a few days. You can actually see on the graph that the traffic is totally flat for a few days which is unlikely.