I don't know who you know on Facebook, but I really don't know anyone who complains that someone's not updating.
Email, phone or text are three different things, when you can basically have all that in one easy clickable spot that you look at that contains all your friends.
I don't need to email them, to have them forward me the url of another website to see their vacation pictures. All they need to do is put 'em up and they're right there to see.
I don't have to pick up my phone and talk to them if they're online, in the same spot I'm communicating wth other friends... they're right there.
I don't have to put out a mass email searching through what names I want (a good number of friends having several email addresses and sometimes you don't know which one is their primary), fill up someone's mailbox or whatever. I just slap up the event on my profile and my friends know about it. If I want to invite specifics it's as simple as clicking their picture.
Old gal pal of mine that I do infact still care about and always will, found me on Facebook. She doesn't have to be a slueth to try and find me in a phonebook, call me, tell me her email address, then forward me a url with the pictures of her new husband and children. She just clicks... and in one shot I can talk to her and see her new family.
Just makes life and keeping in touch with people I care about crazy easier. Cuts out steps that would otherwise make it more difficult. And this is >not< bragging, but I do have a LOT of very good friends that I care about dearly. And when you have several hundred of those, cutting down the steps in keeping up with them and their lives is greatly appreciated.