Thats crazy. The diagram is more helpful than it looks. So its not just for friends of social networking but also for content sharing websites because it changes due to other users?

This is all super true. It's also partially because users self-select information sources on the internet, but at the same time, various content platforms and search engines which give the appearance of providing information neutrally actually tailor results to you. This, in combination with social factors like the article talks about, explains one of the beautiful/awful things about the internet: its enabling of fragmented groups to proliferate. On the one hand, it's awesome -- niche communities can thrive because people with unique interests can actually find each other and build communities. At the same time, it's really easy to get trapped in an echo chamber and not realize how far you've deviated from center.