To me, Facebook is the leading edge of what is possibly the most evolutionary aspect of the web: the transparency of each of us and all of us.
Increasingly, people are signing on and donning a Facebook mask. This mask is seen not just by those one wishes would see it, but by anyone one accepts into one's circle. For most of us that means people who knew us as children, family, current friends, professional acquaintances, etc. No longer can the image you put forth into the world be so segmented. Now everyone sees the same pictures, the same likes/dislikes that you broadcast, the same political views, the same mask.
It's still a mask, but in my opinion, it's harder to obfuscate your true self to all these people who know different aspects of you. Thus, the mask probably tends to become more authentic of our true self, and perhaps we are encouraged to live up to the ideal we want the world to believe that we are.
The second aspect, and Facebook is just one piece, is the clearing house of the collective consciousness that is on display online. I believe that a collective mind is becoming transparent to an ever larger group of people, because we can see that we are experiencing and thinking things in sync. Leaving the mystical aspects of a collective mind to the side for one moment, I think it is a profound step for people to begin seeing themselves as part of this collective cultural force, instead of discrete atoms.
