Looks like Miitomo has surpassed 1 million installs on the Android devices. It is sitting somewhere between 1 million to 5 million times.
the “Top Charts” data viewed from the web version of Google Play currently differs from the same chart data viewed from an Android device. For example, when viewing from my Nexus 5 smartphone, Miitomo sits at the No. 1 spot for the “Top New Free” and “Top Trending” charts, and at No. 3 for the overall “Top Free” chart; the data doesn’t seem to be reflected the same when viewing the web version of Google Play.