Hmm, I know at the secondary school I work at, students have been asking the Head for something similar to this because, obviously, facebook is not accessible at school and, also, most of our under-13s do abide by the age restrictions on fb and don't use it. So, for them, it would be useful.
As for primary, I can see your point about the inappropriate way your dd messaged the HT but, thinking about it, maybe this early introduction is an excellent way to prepare the children for the realities of real world social media - in that you as parents are monitoring it far more closely than you might when she is 13, and the school are too. I can see great potential in PSE lessons at school for addressing all the issues you mention and, really, at 8 or 9, the lessons might sink in much more readily than at 11 or 13.
As for the blogging, I agree that it could be controversial but, again, at least it's monitored. I think there should be some kind of previewing going on by the school before the blogs 'go live', btw. Our yr 8s are all creating their own blogs at the moment but they're not public (even to each other) until the relevant teacher is happy with them/their content/privacy etc.
On balance, I think it's a good thing to address the realities of social media before the children are old enough to be a) very secretive or b) ingrained in bad habits.