Here's a sort of Beginner's Guide to Facebook that's helpful, talking about the basics. The only thing I personally disagree with is making the profile THAT public. Mine's quite private - and very selective. I only accept friends/relatives I actually know.
In terms of promoting our writing, I think Facebook can also become a useful tool. I have a few author friends on Facebook, and it's interesting to see their writing-related posts. But -- I would caution -- if you use Facebook solely as a marketing tool on your personal Facebook page, your friends/family will quickly grow tired of all those book-related statuses. After awhile, it can seem like you're using Facebook solely to promote a novel, and friends/family can see right through that. It can start to seem like you're a salesman/woman, peddling your wares, and soon, friends will ignore your posts altogether. So, I suggest a nice mix - keep things personal, and sprinkle news about your book hither and yon.
I joined Facebook for personal reasons - connecting with old friends. But, sure, I do post occasionally about my writing, getting an agent (and hopefully, if I get published, I'll post about that too). Writing is a part of my life, so why be timid about sharing it? But - I work hard at not boring my friends with too many writing posts. I save those for this blog. ;-)
If I ever decide to get serious about "promoting" my work on Facebook (again, assuming I might one day be published), I think I'll create a separate page, like an "author" page, that's more public, with the sole purpose of promoting the work. I don't ever want to "use" my personal Facebook page to drive my friends/family crazy with pushing my novel (not that I won't give them occasional updates and such -- I've found that when I do post about my writing, they're incredibly supportive, which is lovely).
So, as with any new toy/tool, use it wisely, use it carefully, and always ask yourself how others will perceive you. I think that's a great barometer for figuring out how to present yourself in this age of Social Media.