Interesting study "Do Friends Influence Purchases in a Social Network" from Harvard Business School (PDF). Excerpt from the abstract:
"Our results show that there are three distinct groups of users with very different behavior.
The low-status group (48% of users) are not well connected, show limited interaction
with other members and are unaffected by social pressure. The middle-status group (40%
users) is moderately connected, show reasonable non-purchase activity on the site and have a strong and positive effect due to friends’ purchases. In other words, this group
exhibits “keeping up with the Joneses” behavior. On average, their revenue increases by 5% due to this social influence. The high-status group (12% users) is well connected and
very active on the site, and shows a significant negative effect due to friends’ purchases.
In other words, this group differentiates itself from others by lowering their purchase and
strongly pursuing non-purchase related activities. This social influence leads to almost 14% drop in the revenue of this group. We discuss the theoretical and managerial implications
of our results."
