For Hotmail, it was inserting an e-mail signature at the bottom of each message that turned every e-mail sent by one of its users into a pitch for new users. For Airbnb, it was craigslist infiltration, which allowed Airbnb hosts to use the site as a sales platform. For Groupon and LivingSocial, it was their referral offers that paid users to share deals with their friends. And for Spotify, it was the free “advertising” it got from Facebook integration.
