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.

Then here a couple more key points: the NPD Group said in a statement, "[I]t is worth noting that 10 percent of social network gamers have spent real money playing these games and 11 percent indicate that they are likely to make a future purchase."

Also, "Social network games are also impacting spending on other types of gaming activities as gamers report spending 20 percent less on gaming overall since they started playing social network games."

So this looks to be a profitable space, with a lot of people other than procrastinating college students and bored housewives willing to devote a handful of hours and dollars to simple games.

As a result, Facebook would do well to continue trying to make friends with developers, and as Google (allegedly) builds a social network of its own, it should keep trying to take social gaming into account.