One of the weird things about World of Warcraft was that there are (or were) folks who would sort of hang out in that world just to socialize and sometimes, perhaps, go out and kill some stuff. The 'hanging out' part was key, and I am convinced that if Blizzard could figure out a way for folks to sit down and play cards or checkers or something it would be popular.
People like to sit around and socialize, and if they can do it with an 'excuse' like a game of cards or a table top game, well that is just icing on the cake. I don't think Zynga has quite got the formula down yet but when they do, or someone does, you will see the rise of the 'Grandma chat rooms'.