threshold idea: one standard deviation above the mean

[BrianHicks]

This is pretty close to the current logic, if you squint a little...

Basically, this bot wants to boost the very best toots. Put another way, those are the outliers measured in favs! It'd end up something like:

SELECT id
  FROM public_toots
 WHERE favourites_count > (
       SELECT stddev(favourites_count) + avg(favourites_count)
         FROM public_toots
        WHERE created_at > NOW - INTERVAL '30 days'
       )

2*stddev seems to be a more common outlier function, but it'd only find the really exceptional toots instead of just the uncommonly good toots.

As a bonus, it's likely that stddev is implemented efficiently in the database!