It's not that Facebook is the work of the devil or evil (I don't think anyone's actually said that here?), the point is social media isn't a good idea for young children.
When I was about 10 I was allowed supervised access to social media to keep in touch with family. This supervision relaxed over time as i'd been supervised from 10yrs old and apparently knew how to be responsible online. My knowledge of how to use social media obviously increased too. I joined msn messenger to chat to cousins and school friends, but then figured out how to go onto the msn chatrooms without my parents finding out. Then I searched the web and joined different chat rooms, MySpace, face party, bebo, habbo hotel, etc, and had more than one msn account and email account so my parents didn't know. I'd chat to older men, pretend to be older and would learn about sex from my online 'friends' too. I even met up with a few people I met online (luckily they turned out to be young boys the same age as me) and brought friends along putting them in danger too. My parents trusted me and were oblivious to how easily I could access different forms of social media, but I'd learnt from a young age.
Now with my own son I wouldn't let him even go on child targetted sites like penguin chat, nevermind have a Facebook account from a young age. The longer I can protect him the better.
Before DS started primary school, there was a welcome assembly for parents where they had a presentation all about the dangers of social media, the children are taught about the dangers and the teachers all urged us not to introduce our children to social media when they were young.