I understand what you're saying. For me, though, I do not want my kids using FB as a diary. I don't think, at that age, they fully understand what is appropriate to post publicly and what isn't.
I have a few teen relatives on my facebook. Their accounts and all their friends have me banging my head against the wall from time to time. Posting stuff they really should be more careful with.
A diary doesn't need to be on a public site.
My mom's best friend's grandson, "J" is on my facebook. He was raised by his Grandma because his mom is schizophrenic. She's fully functional, working, doing some schooling, etc but her schizophrenia "surfaced" when J was fairly young and had to be hospitalized for quite some time. It was just better if J stayed with his Grandparents.
Anyways.... J is 17 and knocked up his 16 y/o girlfriend of 2 months.
They friggen announced it on Facebook! That's how J's Grandma found out.