[gtc]

I believe that it was simply a matter of being in the right place at the right time for Zuckerberg and facebook. Friendster, Orkut, Myspace, and I'm sure others that I'm forgetting or unaware of, all paved the way for facebook.
Yes, facebook is a bit more userfriendly, but that was simply a matter of ironing out their UI and perhaps even making it more simple (ala Apple products).
I have to say, I'm surprised Facebook is still around and that there isn't another "new new thing" social network already. Someday soon I think we'll abandon "profile pages" and I wonder if Zuckerberg can pull it off.