  Many of today's most widely used computing applications utilize social
networking features and allow users to connect, follow each other, share
content, and comment on others' posts. However, despite the widespread adoption
of these features, there is little understanding of the consequences that
social networking has on user retention, engagement, and online as well as
offline behavior. Here, we study how social networks influence user behavior in
a physical activity tracking application. We analyze 791 million online and
offline actions of 6 million users over the course of 5 years, and show that
social networking leads to a significant increase in users' online as well as
offline activities. Specifically, we establish a causal effect of how social
networks influence user behavior. We show that the creation of new social
connections increases user online in-application activity by 30%, user
retention by 17%, and user offline real-world physical activity by 7% (about
400 steps per day). By exploiting a natural experiment we distinguish the
effect of social influence of new social connections from the simultaneous
increase in user's motivation to use the app and take more steps. We show that
social influence accounts for 55% of the observed changes in user behavior,
while the remaining 45% can be explained by the user's increased motivation to
use the app. Further, we show that subsequent, individual edge formations in
the social network lead to significant increases in daily steps. These effects
diminish with each additional edge and vary based on edge attributes and user
demographics. Finally, we utilize these insights to develop a model that
accurately predicts which users will be most influenced by the creation of new
social network connections.
