I'll take an example from my game, which is a multiplayer text adventure. This really happened.
One day, you've got a few hundred active users and Everybody Knows that there's a fountain in the centre of town. You didn't write the fountain, it's a thing that players decided was there and started to roleplay around and they talk to each other and Everybody Knows about the fountain.
The following day you get linked from somewhere big and you've suddenly got two thousand people on the site.