ARGs (Alternate Reality Games) take the normal boundaries between games and reality and skew them just enough so that the two overlap, offering a new way to think about entertainment interaction. Perhaps one of the most famous of these is the "I love bees" campaign, which was developed to promote Halo 2. Certainly hundreds played, and part of the gameplay mechanics was taking clues from various places on the Web to complete the puzzles and solve the mystery.
However, is "hundreds" enough to typify the term "massively"? Given that you could play alone and never see another player, only becoming part of a grander, more widespread team, is that really "multiplayer"? If the entirety of gameplay isn't online, does that satisfy the "O" requirement? How precise does the "MMO" definition need to be?