I don't have data, but I'm pretty sure email is all but irrelevant as a social tool from a today's teenager's perspective - something that "only old people use". I'm 29 and it's been years since I've sent a casual email to a friend...
Even for the people I share a house with, we found that email with a mailing list is the most effective way to post notifications to everyone, send reminders, etc.
I haven't come across a serious rival to email for those use cases.
I've noticed the same thing. I exchange emails with a few friends, but it's generally only to swap links for a "read later" purpose (if it was "read now", they'd be sent over xmpp).