The document discusses various online communities that exist outside of traditional social networks like Facebook. It describes communities that formed on Usenet forums and mailing lists in the past and more recent communities that exist on mobile apps, games, and photo sharing sites. Many of these communities are private or invisible to search engines but still engage millions of users in discussions. The document advocates observing what people find interesting in various communities and creating spaces where people can communicate privately and using separate identities and profiles to encourage participation.