the game's quality acts like a force multiplier for word of mouth, essentially you're more likely to promote or talk about a game you enjoyed. But you need some critical mass for that to have any real impact. What good is it if each player refers 1.5 others on average, but you only get to 5000 before the game becomes old news? Most games don't continue to sell forever and the hype will die out eventually, so you want that hype to reach a large audience quickly.