FarmVille, Happy Aquarium, Mafia Wars, and the like are gaining ground. A new report from the NPD Group indicates that about 20 percent of the U.S. population has played a game on a social network sometime in the last three months. That works out to around 56.8 million individuals, which is a rather stunning number. What's more, 35 percent of them (or 19.9 million people) are supposed to be new to gaming, meaning growth will likely continue to take place on a large scale.