[JimmyM]

Looking at Steam, there's a huge undercurrent of games that don't get pushed, don't appeal to the hardcore, but that - in aggregate - sell a very noticeable amount.

Hidden object games, educational games for children, iOS and Android ports.

In the case of the former two - with no real marketing, and with a great deal of competition, these still fly off the digital shelves, and are not expensive to produce. There are people who want these games, and Steam honestly is not doing the best job of getting in touch with these people. Integration with the rest of the Facebook ecosystem would really really help push these games towards the people who want to buy them.

For the last option, people often buy these games when they felt good about playing the iOS/Android game and want to continue the experience (or in rare cases, play a more full-featured version - usually the PC port is incomplete, but sometimes it does contain unique bonuses). I imagine Facebook would have a killer app if it could sync your iOS/Android saved games (or even just achievements) to the PC version of the save game.

TL:DR; I think there's a huge opportunity for a major casual gaming platform, and I think Facebook's pretty well placed to be that platform.