I find an enormous amount of hostility on HN towards facebook, even going so far as a lot of people proudly mentioning that they will never touch it. I don't see why it gets such scorn, I think its a wonderful site.
Some notes on my usage, though:
All of my privacy options are on the lowest possible setting. I treat anything that occurs on the site as if it were public. I don't see why not, I'm not going to pretend that photos of myself or my wall postings are anywhere near interesting enough to hide. In fact I'm not sure why people who put things on facebook want privacy at all. I never worry that something I say might be picked up on by the wrong person because I'd never say anything that I wouldn't want the world to hear.
To a shy person, the usefulness of the site is astronomical.
Among others I am friends with my boss, my mother, my little cousins. Facebook lets shy people like me keep in touch with a massive amount of people where I can write them the modern equivalent of letters very quickly and easily, as well as let them broadcast their life's updates to me. I can keep in touch with all manner of people. I love writing letters, people love getting letters. Facebook is not far off.
Without facebook, I'd have no idea cousin X is having a baby, or that Y is having apartment trouble that I can help them with, or that Z got a new game we can play together, or person A is considering selling their car, and so on.
If I meet someone at an event and really hit it off (romantically or not), I can go on facebook the next day and look them up by name and add them. No exchanging phone numbers or emails or anything like that. I just search for them and find them. In college it was enormously useful for making friends and I still find it useful now that I've graduated.
I made a page for my hometown (90K population city in NH). I broadcast events going on around the city (fireworks, beer festival, city meetings). In this way I help my community learn about the goings-on of the town. It's a surprisingly popular page (more popular than the local newspaper's facebook page).
I could go on, but this is probably enough to give those who don't use it an idea of why I enjoy it so much.