You could quite easily use a process like the one described in the article to select the 'top' 1,000 Python packages, and then scan each of those for occurrences of calls to each of map, filter and reduce, and that way build up a fairly reasonable picture of how commonly each function is used.

While I use reduce a lot, I would guess that even with comprehensions making most uses of map and filter obsolete, that reduce is still the least commonly called of the three.