  Social feedback has long been recognized as an important element of
successful health-related behavior change. However, most of the existing
studies look at the effect that offline social feedback has. This paper fills
gaps in the literature by proposing a framework to study the causal effect that
receiving social support in the form of comments in an online weight loss
community has on (i) the probability of the user to return to the forum, and,
more importantly, on (ii) the weight loss reported by the user. Using a
matching approach for causal inference we observe a difference of 9 lbs lost
between users who do or do not receive comments. Surprisingly, this effect is
mediated by neither an increase in lifetime in the community nor by an
increased activity level of the user. Our results show the importance that a
"warm welcome" has when using online support forums to achieve health outcomes.
