The best ones might seem like innocent little time-wasters, but games like CityVille and FarmVille will pull in revenues of $5 billion in 2015.
"250 million people play games like Zynga's CityVille and Farmville on Facebook every month," says Pietro Macchiarella, analyst at Parks Associates, valuing 2010's social gaming revenue at $1 billion and predicting a five-fold increase by 2015.
It's CityVille and FarmVille that account for the lion's share of those 250 million monthly active users (MAU) - activity tracker AppData pegs them at a combined 140 million.
Zynga is certainly the current top dog, controlling the five most popular Facebook games (184 million combined MAU).
That's made their social games all the more appealing for marketing departments - hence the appearance of items from companies like McDonald's and 7-Eleven within FarmVille.
Whether Zynga will still be king of the castle in four years' time remains to be seen, though while Zynga is busy forming promotional partnerships, other companies prefer to buy up Facebook developers in order to embed their own brands deep into the social gaming landscape.
Disney's acquisition of Playdom soon resulted in ESPNU College Town - ESPN being another Disney property - while Electronic Arts snapped up Playfish, which now produces EA crossovers ( Madden NFL Superstars, FIFA Superstars) along with more customary Facebook fare.