I haven't done any kind (not even superficial ) of research on this but from the little I've seen it seems like the younger generation is using FB more as a 'reply all' or 'send all' instead of using email. I've also seen it used for messaging by kids. The people I've seen using it for traditional journalling or as a soap box tend to be older than (approximately) 30.

It occurs to me that parents have probably feeling pretty protective of their kids for a bunch of years now with respect to the dangers of over-sharing and online stalking via social media and it's turned kids away from that kind of committed/permanent record. The kids are using chat rooms, IM, and Twitter.