There are two main groups of people in the world: those who want to be known, and those who want to remain unknown.
Public figures, movie stars, and CEOs all fit into the former category. It's fairly obvious to tell these types of people: they are most often outgoing, friendly, driven, and popular.
The latter category of people can be harder to distinguish. Nerds, photographers and librarians can all easily be part of the unknown group. Those obsessed with Second Life most definitely are (although they are often channeling their lifelong desire to be outgoing through their Second Life avatar). But really, anyone who likes to stay behind the scenes desires to remain anonymous most of the time. I should know; I've been this way my entire life.
The downside to being known is that you have a lack of privacy. This is most obvious with celebs and public figures but can be true for anyone who puts themselves out there and has some sort of success.
The downside to being unknown is that when you have something to say, no one is there to listen because no one knows you exist.