According to Blizzard Entertainment, Hearthstone: Heroes of Warcraft has surpassed 50 million players worldwide, just as the most recent expansion Whispers of the Old Gods makes its debut today.
The popularity of the game has been increasing leaps and bounds specially in the last few months. Earlier in February, Blizzard said more than 40 million people were playing the game, from May 2015 when it had 30 million users. In less than three months since reaching 40 million users, more than 10 million new players joined in on the fun.
[Source]: Blizzard Entertainment (via Business Wire): Whispers of the Old Gods™ Takes Hold of Hearthstone® as Registered Players Top 50 Million.