Generally, stuff like this is measured in monthly users, as those numbers are much more reliable. This drop shouldn't come as that big of a surprise. There was a time there where Pokemon Go was beating out Facebook and Twitter in terms of daily engagement, and nobody in their right mind should have expected that number to keep up. Niantic almost certainly didn't, which is why they immediately got to work developing new content for the title.
Pokemon Go is still the number one free game in the Google Play store and number two free game in the App store, so its current popularity isn't really in question. It's its longevity that's hard to predict, and we're still way too early in its life-cycle to get a good idea. It hasn't even launched in all territories, yet.
Even if the numbers continue to drop, expect them to rise again as Niantic adds more features, namely the ability to trade Pokemon.