The problem with a public diary - a blog - is that you feel you need to conceal parts of your life, 'cos anyone can see them, including people who know you. You feel you can't reveal negative thoughts or fears about them, or stuff you do... just in case they find this page. Even though that's a long shot, you can type in my name on a search engine and find the Blog Cabin.
So net-based stuff tends to be recorded here, and a 'public image' is created. Practically nobody is looking at it - but it feels public. I've got stuff going on in my private life - new girlfriend (I think :) ) for example - and in my PnP diaries that'd predominate. Here I want to keep that shit to myself. I've got a bunch of students who use the net, for Christ's sake.
Maybe it's a good thing. After all, diaries tend to be a litany of misery and self-doubt, written to no-one, so sentimental and empty of content. At least with an awareness of a potential audience - even though they're probably absent - you feel the need for a certain quality of content, a certain self-editing, a will to create readable stuff, rather than, well, merely waffle.