My DD has just turned 13 and has had a phone since she was 11y, ready for starting secondary school. She knew from the start that the agreement of having a phone with email, test and social media was that I had full access to all, and all her passwords. I was also to be her "friend" on social media. She knows that she should only write things she would be happy for her mum to see - and that I am techy enough to see if/when messages are missing, etc - it is obvious when something is missing ime.
I don't check that often these days tbh. Can't remember last time I felt the need or did do it. It was always inane daft stuff mainly or just chit chat with mates.
However, by having that agreement it does mean that if I was ever concerned I could check and not feel bad.
I disagree with those who think it is wrong to look and actually pretty much all the online e-safety campaigns, etc do suggest parent having some control and being very aware.
And yes, they could set up secret ones - but again, we have had serious talks about this and the consequences of doing such. And as I have access, and a jointed iTunes account, it'd be quite tricky.
I also have a lot of e safety chats with DD and she is pretty clued up. She also teachers e safety to infant school aged children once a year - currently doing it all week this week infact - so feel she is quite aware of what why, how, etc.
Re privacy - well, she can do that face to face with her friends. She seems them most days. But then she trusts me to not be going hunting unnecessarily and I trust her to be sensible and open with me.
Obviously as she gets older this will need to be readdressed - but 13y is still very young.