Thank you! It is monthly active user count (MAU) divided by number of subscribers. That gives a good metric for active communities, with a preference for newish communities with a lot of organic activity.
A few times a day, it takes the highest community by that metric, that has at least 50 subscribers and hasn't already been posted, and posts it.
I realized that it's pretty easy for the bot to just subscribe me to any good-sized community from an Mbin or Piefed server, and then get MAU counts from the Lemmy API. I've done that now. It's a bit of a hack, but communities from those instances are included in the list as of now.
I'm pulling data from lemmyverse.net, so it is only Lemmy right now.
I just tried to add Mbin, but it looks like Lemmyverse doesn't have the same MAU counts for Mbin that it does for Lemmy. I like the idea of adding it, if I can get it. Do you know of a place that summarizes MAU and subscriber count for those platforms?