Social health games can drive healthy behaviour. To track social behaviour change in social network games (SNGs), gameplay metrics should quantify socially-engaging gameplay behaviour based on player interactions. We developed social player metrics in a quantitative study of player behaviour in a social health game called Healthseeker (developed by Ayogo Health Inc.). This Facebook game targets people with diabetes to help them manage health goals in real life. Our metrics identify which game mechanics led to more gameplay success, connectedness and virality. We also identified how the behaviour of successful players differs from unsuccessful players in the game. Our results support that game mechanics aiming at social interactions can motivate players to solve more missions, to fulfill more healthy goals and to play the game longer. We conclude that having a well-connected social network can improve player success in solving game missions.