The slides below are from Google researcher Paul Adam's recent talk "The Real Life Social Network v2".  Paul's slide deck is a bit long but well worth the time.  It's especially good if you build web apps or are interested in how human behavior is influenced by social networks.  Paul deserves special praise for leaving out all the technical and social media jargon and providing human examples to illustrate the important ideas. 

• Differences in how people socialize online and offline focusing.
• Shortcomings of existing social web models.
• Role of influence and influencers in social behavior ... both online and offline.
• Why people care about online identity management.
• Privacy.