Katwick
Cartographer
Helya indicated that logins are currently the only "activity" metric. See:City Relocation is a concern as is trading and neighborhood visits.
https://us.forum.elvenar.com/index.php?threads/city-movement.28895/post-252898
While logins do indicate that the City isn't deserted, I think the number of recent non-Fellowship Notifications would be a far better metric for neighborhood interaction/isolation.
As a notification is generated whenever somebody visits your city and/or pick up your trades, it would be a simple, but very relevent, metric for a healthy neighborhood.