I must say, in my one year on Facebook, I had not seen one of my 200 or so "friends" ("friend" used in the loosest interpretation of the word, more like "positive acquaintances/some friends/people you don't like, but they friended you anyway"):
a) Give or receive cyber-bullying
b) Get in any kind of danger as result of giving away too much information (at least, to the extent of my knowledge)
I have however seen:
a) Some rather curse-filled statuses (but seriously, if your kid is old enough to have a Facebook, than he/she should be old enough to know what the f-word is, and using it is not the automatic ticket to coolness)
b) Horrible, horrible grammar of the first degree.
c) A few statuses of a "questionable" nature (once again, if your kid is old enough to get a Facebook, he/she's old enough to know what those words are), hardly any photos though.
Not saying that no bad stuff ever happens on Facebook, but I'm just going by my experiences on the site.
I have, however:
a) Been able to keep in touch with friends that either go to another high school or even live in another state so much more easily than I ever could with a phone.
b) Been able to arrange/schedule "hang-out seshs" (hip terminology there for you grown-ups) mush quicker/easier than I could even dream of with my cell phone.
So essentially, I find that the whole Facebook/social media scare is rather pointless and created by people who simply don't understand that something was not made with the intention to make kidnappings easier, but I guess that's how it always is with a new technology.
So, yeah, I believe that if your kid seems healthy socially and is 13 at least, he/she should have a Facebook, considering it's probably going to replace the phone as the way most teens communicate, in my view at least.
But what do I know? I don't friend the "morally corrupt" kids or the "stupid" kids (for the most part) on Facebook (and your kid probably shouldn't either).
Oh, and "Facebook addiction" (we throw around the word "addiction" too much as a society) is much more rarer than it is made out to be. I find that for the most part, for every kid that spends half their non-school waking life on Facebook, there's one kid who checks the site for 30 minutes a day, at the longest, and still retains a pretty good social life outside of their computer screen.
So in summary, Facebook is a pretty cool site. Not the best out their, but good when used for it's purpose. Best used in moderation. And it's negative aspects are over-inflated by the media.
But once again, what do I know?