Activision Blizzard (Nasdaq: ATVI) is also riding the network effect to wild success with World of Warcraft. Without millions of users, the game would be just another title on a store shelf, but the interaction with other users makes the game even more attractive to new players. In the process, Activision is laughing all the way to the bank. Competitors like Electronic Arts (Nasdaq: ERTS) have to hope that blockbuster games live up to their billing, leading to a boom-and-bust cycle. But Activision has found a way to capture a constant revenue stream.