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)
I couldn't imagine being a moderator there, the amount of shit they must see uploaded has to be enormous. This would apply to every media-oriented instance but due to their nature I am guessing it's worse