I use facebook essentially as a phonebook. If I ever need to get a hold of someone, I know where to find them quickly. Besides that, I don't use it very often.
But Twitter boggles my mind. The only time I could see someone caring about your daily updates is if you are at least relatively famous, are giving updates for an organization, or are extremely interesting. Besides that, I'm not sure why people would take the time to tell the world about their daily lives.
And something that worries me is the reach of search engines into facebook and other social networks. For a while now I have had my profile set to private so only my friends can view anything about me... but a while back I googled my name and what do you know, a friend of mine didn't have her profile set to private and a comment on her wall by me showed up in the google search. I Emailed Google at least 10 times over many months before the pages was finally removed for their cache. I am very conscious of the fact that employers google people now... and I'd like them, and anyone else I do not know, to know as little as possible about my personal life.