Remember Cow Clicker? This great Wired article by Jason Tanz is worth a (long) read:
And then something surprising happened: Cow Clicker caught fire. The inherent virality of the game mechanics Bogost had mimicked, combined with the publicity, helped spread it well beyond its initial audience of game-industry insiders. Bogost watched in surprise and with a bit of alarm as the number of players grew consistently, from 5,000 soon after launch to 20,000 a few weeks later and then to 50,000 by early September. And not all of those people appeared to be in on the joke.
See also: The Life-Changing $20 Rightward-Facing Cow.