I don't know what the usage stats are like, but those factors are probably worth a lot to teenagers who either have to pay their own phone bills, or have to convince their parents to do it.
I have never in my life written or received an email from someone in my age bracket intended to communicate. When my classmates and I use email, it is to share attachments or to communicate with adults.
iMessage is just slightly nicer SMS. Very common in Apple-heavy social groups and not that useful in others.
It's almost like 13 year olds are people too and don't operate as one homogenous block. I really think it would be better for everyone to stop pretending that they understand what EVERY 13 year old needs, wants or does.
How many teenagers today will look at their instagram comments as a 21 year old and consider anything other than a vaguely interesting window into their former naivety?
Obviously there will always be exceptions. This isn't really about teenage prodigies publishing great works though, it's about teenagers chatting to each other.
Like most Facebook status updates? :)
The end.
The aspects that I use facebook for most - keeping in touch with fairly widely spread social groups - are not really relevant to a 13 year old. I don't think I'd have needed anything as complex as Facebook when I was 13. 13 year olds like to message each other, be it via SMS, IM or some social networking platform du-jour. Their messages are disposable nonsense that don't really need anything like the complexity of Facebook.