I asked: Aki Järvinen (Finnish game researcher) about social games:

@aquito what’s the difference between “online multiplayer game” and a “social game”?

His reply was:

@gameproducer No definitive difference but leveraging a pre-existing network of friends on a social platform for play differentiates ‘SGs’

For more definitions, see yesterday’s blog post defining social games. Don’t forget to check the comments too.