There are several ways to know if people are using TeamMood on a regular basis.
Usage overview over the past 30 days
From your Analytics view, take a look at the Usage Chart to check how many people used TeamMood per day:
It shows the number of registered users, the number of those people who shared a mood, and how many of them has left a comment.
Weekly participation rate
You can get an export of the weekly participation rate for a team from your Export Center.
Each line is the participation rate for a week, from the beginning of TeamMood usage:
You'll get an overview of the usage trends from your Periodic Digests that you get by email or Slack etc.