I can see that at 11/12 it's something that can be useful for maintaining new friendships when moving secondary school, but that is only dependent on whether there is total access to the child's account and firm rules as to useage - eg i wouldn't let my son use it alone in his bedroom.
Even still though, i'd be very concerned about the material he will be exposed to - language, photos and videos of other people that come up on the news feed. Therefore i won't bring it up with him until he mags me for an account, and even then i'll stall him for as long as possible.
My friend's son is 8 nearly 9 and had an account at least a year, probably more. He saw a video on there and showed it to my son aged 5 who was very distressed (the monkey who ripped off the woman's face, and another was a rocking chair then a scary face i beleive....i can't bring myself to find them and see what must've upset my son). I was furious about it, but my friend's son saw it on FB and is obviously desensitised to all tht. She gives him unchecked time on her iphone too, so he goes in search of alsorts of videos. I think she's really got it wrong - he's far too young. She hasn't a clue how inappropriate FB is for her son, and was dismayed with his primary school for writing and requesting all kids at the school with an account have it deleted....she just said he didn't have one, so now she's encouraged him to be dishonest to teachers.
She thinks that her son has been helped enormously because he's typing more and spelling more whilst using the chat function, and who am i to disagree with that?
Such a mess if you ask me.