The market researchers from Survey Monkey are calling Pokémon Go "the biggest mobile game in U.S. history". According to the researchers, Nintendo's mobile game beat Slither.io and Clash Royale within 24 hours to become the biggest game of 2016 (as measured by daily active users). Within three days of its release, Pokémon Go rose to the top of the App Store revenue charts, earning millions of dollars per day. Yesterday it attracted just under 21 million daily active users in the United States alone, surpassing Candy Crush Saga, which is said to have had around 20 million users a day during its peak.
Update: The original link to the Survey Monkey research is actually broken now, but the folks over at FieldBoom have prepared some interesting data that speaks to the popularity of the game and offers an overview of its popularity.