Imagine if you had the capability of examining your great-great-grandfather's life at the daily level. How amazing would that be? To localize this, imagine yourself in your 80's being able to zoom to any day of your life at any point to relive and review how you thought, what you thought at that particular moment.
This is what I use Facebook for. Sure, I use it to connect with my friends, but I also use it so that when I'm knocking on death's door, I'll have something concrete to look back at that is more stable than my ailing memory will be. I'll also be able to hand it down to my spawn and their spawn.
I've always considered Facebook a new kind of public utility, just as revolutionary as the post office used to be.