A police friend of ours has been into DN's profile and put together a report of everything he has gleaned from it about DN.
DN has a list of friends who are all visible, the names could be used to narrow down her location in the UK and also as a reference when approaching her in the street ?Hi DN I?m [friend]?s uncle? for example.
He now knows some of her likes and dislikes and thinks she's 14.
He now knows what her dog looks like, its name and that it looks just like [friend]'s dog.
He knows her close relatives are called [name] and [name] and they fight.
He has listed her status updates and that she has just had a holiday in [place] and listed some of the clothes she was wearing in her photos.
DN mum is called [name] who is married to [name]. He has her DSD work history and where he is currently working.
He could use all that ?Hi DN, I used to work with your mum?s husband at [company name]?.
With all that information he could use websites such as 123people.com to find out more information and pay a small fee to obtain DSis' and her DH's address. Or even simpler he could just follow her DSD from the office to his home, where DN lives.... 
All information obtained in 20 mins using nothing other than information on an unsecure facebook page.
DN has only been on facebook for FOUR DAYS! And all that information was available to the public. Which clearly demonstrates that 11 is too young for an account. If they cannot keep themselves safe, they shouldn't be on it.
Giving an 11 year old a facebook account and justifying it by keeping a close eye on it yourself, is like having to hide in the bushes when your DC is at the park on their own. You don't let them go to the park on their own until they are aware of 'stranger danger'. You don't rely on other people to keep them safe, you teach them to keep themselves safe.
Unfortunately there are too many parents that just don't understand the dangers and intricacies of social media and that puts their children in danger.