that's why facebook was so obsessed on getting new users to connect with 10 friends in their first week. they discovered an extremely strong correlation between this and long-term retention. classic example of what drives network products
Discussion
The fact that you had to interact with friends -- and that you would only get into Facebook because you already knew friends who were there (and then they would take data from your email contacts and then your phone contacts) made that onboarding simpler.