It's discovery/referral gating. Teens who get Snapchat upon hearing it from a friend can ask their friends on how to use it, and it stimulates conversation and

It's discovery/referral gating. Teens who get Snapchat upon hearing it from a friend can ask their friends on how to use it, and it stimulates conversation and feature discovery. If you don't have at least one existing friend who is experienced at using Snapchat, you're probably not in their target market (by design).