I haven't read the whole thread, yet.
DS1 doesn't, to the best of my knowledge, have a diary. I wouldn't read it if he did. The only situation in which I can imagine reading a diary is if he simply disappeared, and I had no idea what was going on.
When he got his cellphone, we were paying for the plan. Canadian cellphone plans suck (I hear this from everybody, although I have no basis for comparison) and he had a very rigid limit on texts (he was 15), because we weren't going to pay for them. But, I've never read his texts, unless one is sitting on the screen, and I happen to sit down next to the phone (think it's happened twice in three years - both were utterly innocuous..."what's up, bro?" type stuff).
I don't go through his room and never have. Since he turned 12 (when we moved in here), the only time I've ever gone in his room without checking is if I needed to dump his clean laundry in there (he has a bad habit of leaving it in the dryer), had to let in a service person (only once) or he left it unlocked and I had to go get one of his siblings. I do have a key, though.
Up until he was about 10 or 11, I went through his backpack sometimes, but only to look for leftover lunch that he'd forgotten about, and/or notices or invitations from school He was kind of airheaded.
When he got an email account, I had his password. But, he opened a new one about a year ago, and I don't have the password, nor am I on his Facebook friends list. He's 18, and is only still at home, because he can't afford to go to school and pay for a place. I have no need to pry into his life.
I'd probably be somewhat less rigid if I had any reason to be concerned about his behaviour, but I don't. He's always maintained a good standard of behaviour, has never even flirted with drugs and had one drink (not even a whole one) at a party once. He has a really solid group of friends, and chooses to hang around with people who are good for him, not those who aren't. And, he was born with more self-confidence than I've ever had, and is more than capable of ignoring negative people in his life. So...why would I poke around, yk?
We'll see how things play out with my other kids. I can see a possibility that I may end up prying a bit with ds2, as he has some behavioural issues that are a potential concern, and probably also has some kind of special needs. I don't know what the picture's going to look like when he's older, yk?