I'd say it depends on the kind of content you put in your diary. When I was a teen I used to keep a diary with code names for anyone I mentioned. My little sister stole and read it, and though there wasn't anything too detailed in there, I felt exposed and angry.

Years later I started writing my thoughts down in a blog. Sometimes just making funny observations about my day or incidents that happened to cause me joy, upset or the need to moan. I didn't know anyone was reading it until months later someone commented about how much they enjoyed reading my posts as they drank their morning coffee. Again I felt terribly exposed, but more people started encouraging me to the point of complimenting my gift for writing. The positive result was, I eventually wrote my first novel.

Now though, I feel like I have no personal outlet for me to write down my most intimate thoughts that wouldn't somehow harm my reputation. I don't like book or phone diaries, because they are easy to lose or for people to happen upon. It's bad enough that I keep notes for my storylines in book format, but then it assists me by being evidence of copyright ownership.