Hi all,
For what it's worth, I took a fairly exhaustive look at methods of measuring the use of free/libre open source software and found a relatively weak scientific record. Some research uses browser identification or downloads, or rougher signals like what's in various containers/clouds, what's where in the dependency chain, how many 'stars'/followers in revision control systems, etc. An opt-in automated survey like popcon is quite good, especially given the competing approaches.
What is the ultimate goal of the measurement task (i.e. this is a classic response from an academic -- it's always complicated to measure things, and what's your research question :D)?
But, truly, the best estimation strategy will vary by the goal.
-Kaylea