The basic thesis lies in this: the more we define ourselves in words and pictures, that we then see, the more we are creating a "mirror" that is not reflective, but instead reflexive, ie, representing a momentary understanding and/or desire of who we "are". In particular given than this mirror self has a life of its own beyond our control -- ie, when we are offline -- the avatar/profile/etc begins to act as a stand-in or double for us, and operates in relationships with others, who respond to our avatar increasingly instead of our offline, mutable selves.
When we return to our online communities, etc, we come into contact with the relationship our avatar has to others and to their doubles, and we often slip behind the mantle we have created for ourselves to perform along pre-defined lines, using certain language, making certain references, and now doing allotted actions per facebook such as "poking," which now define certain mores and modes of interaction previously unknown.
How does this affect who we are when we return to the topic, ie, "real"/physical space? As many of us have long been familiar with Gamers, both those of the new online variety but also those who played Role Playing games, or engaged in character based convention activity (ie, trekkies) we see people who are itchy to return to the "stage" in which they can play their character, which becomes more real to them then the undefined lived self.
Through this move towards online social interaction via avatars/doubles, we are increasingly simulating a "game-state" in the way we see our "selves" -- we are become characters/caricatures that are more familar to us then the changeable, variably reactive humans we are forced to remain offline.
This condition is reinforced by other societal shifts towards isolation, where capitalist driven urban planning has created a landscape in which we are physically isolated from eachother, and various technological "developments" have brought this alienation into even the most crowded of cities, in which we have individuals iPodding themselves against the world. A trip to nearly any global city now displays a similar seen: in crowds, individuals and groups listening to, playing with, and or looking at devices that now more than ever draw our exchanges, once natural and based on environment, back into pre-defined territory, partially by the self and partially not.
Why should we wander? We have garmin. Why should we meet new people? There's twitter. The list goes on and on, and the game-of-our-lives is becoming more mobile than ever. Watch out... soon we won't bother with mirrors anymore.