Blizzard Entertainment's free-to-play collectible card game Hearthstone: Heroes of Warcraft has reached a new user milestone. The game has now seen 30 million players, according to a tweet from the game's official Twitter channel on Tuesday.The user milestone was achieved following two major Hearthstone developments in April: the release of the Blackrock Mountain expansion and the game's arrival for phones.Being a free-to-play game, Blizzard doesn't make money from installs alone.