Social Gaming is a pervasive phenomenon, driven by the advent of social networks and the digitization of game dis-tribution. This paper positions and defines Casual Social Games (CSGs) as a genre and platform agnostic subset of Social Games that incorporates browser, mobile, console and wearable digital games. The authors argue that – as CSGs impact the games industry, shape play patterns and audience characteristics, and proliferate to new platforms – understanding and measuring their social aspect becomes highly relevant. A randomized experiment on added social gameplay in a CSG on both mobile and Facebook serves to support this argument. Experimental results highlight that social gameplay is extremely important for engagement and monetization in casual games, even more so on mobile plat-forms. This does not only suggest that CSG developers will benefit from focusing on increased social interaction in their games, but that Game Analytics should strive to unify defi-nitions and build a common body of knowledge around the social aspect of casual gaming.