My previous post grew out of control, so I hacked it in two.
The effect need not be limited to a well-delineated work: it may extend into other media and spheres, and even merge into what we think of as reality. This can turn into a kind of decentralized work of art / subversion, where people who do not know each other contribute and it becomes impossible to tell hoax from reality. Think Hakim Bey.
Some games have tried to do this: The Light Files, written and designed by Lee Sheldon, is an "Interactive non-linear online mystery game. Investigating the case of an impossible murder committed during the taping of a daytime soap opera, players pursued the story through clues uncovered at multiple websites." It was online from 1996 to 1998. And of course there is EA's intriguing but unsuccessful Majestic. Stéphane Adamiak told me about another game like Majestic, but I've forgotten the name.