Active users is the standard metric used to check how much a service is used (at least as far as i know. its what i see when i look at stuff published for investors).
hexbar is on the sixth place in term of number of active users with 1.8K , lemmy.world is 18K (enable the "active users" column and sort by it to see the full list)
In the future i think there should be a nice store with reviews like wordpress plugins or discourse plugins .
Having a great UX which will motivate people to extend piefed (like firefox addons website) can be really a killer feature .