My rule with my kids is if they keep it private then I respect that privacy. I wouldn't read my daughter's diary unless I had very very good reason (found hard drugs or evidence that she was sexually active at a very young age ect). But if they think they care share it with all their friends then it really wasn't that private.

Not supervising your kids doesn't help them to develop independence and confidence it allows them to make life changes mistakes during one of the most vulnerable times in their lives. Once they go off to college they will have all the freedom they can handle and they might finally be old enough to handle it. The problem I have with the argument that kids will better be able to handle adulthood if you give them the rights of adults when they are teens but I have to admit I don't think the results prove that out. Kids today get much more freedom then kids in my generation did when we were growing up but I see no evidence that they handle adulthood better, the opposite is more like it.