Less than a day after it launched, the location-based mobile game Pokémon Go from Niantic and The Pokemon Company is now the #1 most popular free iOS app on the U.S. App Store.

It's also the top grossing iOS app, supplanting (at least for the moment) proven earners like Mobile Strike and Game of War: Fire Age, which means Niantic's strategy of distributing the game for free and selling in-game currency and items for real money seems to be working out well.

Intriguingly, the game was simultaneously released on Android and while it seems to be doing well on the Google Play store, it's not currently charting among either the top free or top grossing Android apps.

The game is currently available in select regions, including the U.S., Australia and New Zealand, and it's expected to come to Europe, Canada and South America in the near future. Given Pokémon Go's remarkable early adoption here in the U.S., it seems likely that Niantic founder John Hanke's ambition to successfully distribute a Pokémon game in regions where Nintendo hardware is typically rare will pan out.

"There are markets where they don’t even sell Nintendo hardware, because the price point and distribution doesn’t work out -- India, for example, or Brazil -- but there’s surging smartphone usage," Hanke said last year. "Pokémon may be known through the animation or the cards, but people haven’t had the chance to play the games before. They’ll be able to play the games for the first time."

Nintendo's own inaugural smartphone app, Miitomo, saw a similarly meteoric rise up the app store charts when it launched in March. However, interest (and downloads) quickly declined due to what some analysts perceived to be a lack of opportunities for meaningful player engagement.