At personal goal-setting websites, people join others in committing to a challenging goal, such as losing ten pounds or writing a novel in a month. Despite the popularity of these online communities, we know little about whether or how they improve goal performance. Based on theories of goal-setting and group attachment, we examine the influence of two social factors in an online “songwriting challenge” community: early feedback evoking a shared social identity, and one-on-one collaborations with other members. Combining five years of longitudinal behavioral data with member surveys, we find that users who engage in these social features perform better on their goals than those who are non-social. Furthermore, these early social experiences are associated with strong community-centric behaviors in the long term, including donating money and providing feedback to others.