A fun site that allows you to keep in touch with your friends and post information about yourself. However... the user-uploaded content can contain nearly anything, and is almost never reviewed by moderators. Use with caution on unfamiliar pages.
Also, there appears to be some ethical issues with Facebook selling information user's information without their permission, which violates the Terms of Agreement. Bottom line: Don't post anything that you don't want sold.
Facebook does not take privacy seriously. They are more than happy to hand over all sorts of info to advertisers, and unless you use some sort of ad blocking software and some tweaking, the ads are annoying.
You've really got to be careful opening any things on Facebook...a lot of it can contain a virus. I've had issues with privacy...I had my cell number posted on my wall for all to see, something that did NOT do myself. I have my privacy at the highest settings possible. Be Careful.