Just because a person is into fiction/fantasy or technologically smart doesn't make people a nerd. 20 years ago, if a person spends a lot of time on computers, they were considered nerds. Now that with the instwebs being an integral part of social networking, the few who think they are into "nerdy interests" can now share with people with same interest.
But to be truly a nerd by definition, you have to be socially inept or lack interpersonal skills to interact with people who follow mainstream. The generalization comes from that nerds are very accepting of social outcasts because they are so themselves. And if you don't look a certain way and for some reason cannot gain acceptance from the "cool people," then you turn to people that will. And sooner or not their interest becomes your interests. Ergo the circle of nerditude.