Holy crap, you people really like playing Pokémon. A combination of nostalgia, interesting augmented reality gameplay, straight-up social novelty, and (not least) being a free game has made Niantic's Pokémon GO rocket to 100 million downloads in the Play Store in just a month. Life isn't a race, but if it was, Pokémon GO would be so far ahead of every other game that they'd see it disappearing beneath the horizon.
It helps that the generation that rocketed Pokémon to the top of video game sales charts in the late 90s is now in their 20s and 30s, and pretty much all of them have a smartphone that can download the free game in a few seconds. It helps even more that Niantic has been tirelessly expanding the game's availability: across all of North and South America, the biggest markets in Europe, Australia and New Zealand, Japan and Hong Kong, and just this weekend, Southeast Asia. At this point the Middle East, Africa, India, and Korea are the only huge markets left waiting for the game's official release. Oh, and the game is banned in Iran.
Niantic has also been working the kinks out of the initial release at the same time. A major update improved iffy performance, mostly by removing some server-heavy tracking features and blocking third-party tools. The blowback after removing features from the app was severe among the initial hardcore players, but a recent increase in community interaction on the official Pokémon GO social accounts has quelled the ranks. Returning features, plus new stuff like sponsored locations, should come in the future.