Your point occurred to me the other day when measuring number of downloads of whole packages was mentioned as being the ideal measure.
I have more confidence in measuring the contents of the packages themselves as creating a Hackage package suggests a competency with the language and baseline of sophistication.
My thought was the best measure might be the count of functions across all packages, where an included function in any individual package would only be counted once per package.
In this sense, map and fold would still likely be towards the top, but that is as it should be.
But it seems there are many measures that would be useful, like the percentage of functions from a package that tend to get used in the same project - do people nit pick particular functions from a specific package or does the package use tend to require the use of all of its functions in every project the package is used in.
I'd love to hear some thoughts on this as I generally don't know where to begin in solving these sorts of problems and would like to know more about those methods in general.