It is often desirable in software-related grant proposals or progress reports to include quantitative measurements of software usage, so access to such tracking numbers for Q2 plugins would be a really nice feature for developers.
I don't know how anaconda counts downloads but for rough numbers you could use this page. Assuming that the majority of users install via conda. Files | Anaconda.org
@benjjneb, another idea (this may originally have been @BenKaehler, @Nicholas_Bokulich, @thermokarst or @ebolyen's idea, as we were discussing this last week) would be to compile some information based on the dada2 forum tag. The anaconda download counts are probably overestimates (I think this probably includes the downloads that occur every time the build system installs QIIME 2, among other things), while forum-based counts would give you an underestimate of how widely used it is (since a lot of people refer to forum posts, but don't post themselves).
Just a quick note on this - the anaconda counts are most likely not overestimated from the perspective of QIIME 2's build/test CI systems. The build system (busywork) and travis-ci both point at an entirely different anaconda.org channel. That doesn't mean others aren't using our official release packages for their CI/build systems, but I suspect that number is actually quite low. From my perspective, the anaconda.org download stats are the best estimate we have, although they are still (probably) slightly inflated (but probably not by a significant amount). Also worth noting, there is a rough OOM estimate for Docker pulls up on Docker Hub, but the stats aren't granular (no per-release, per-package, etc.) - currently ~10K image pulls.