An alternate reality game (ARG) is a kind of narrativist meta-game which creates an interactive adventure for many participants to solve. ARGs are usually created by planting clues in websites to find riddles or puzzles, the solution of which uncovers more of the fiction. They are played as mass cooperative exercises, and often funded by brands looking to engage players in their world.
As with all meta-games, ARGs tend to lack robustness and have issues of synchrony, but they also breed strong subcultures. So while they are intended to be played by thousands of players, in reality they are played by very small groups (under 100 is not uncommon) of very committed players.