Getting the balance right is difficult. Jim is absolutely correct, it's the personal that leads to engagement, and this is a very different voice from the objective one of academic papers (which is not to say it's better or worse, just different). So you need to put an element of personal in there, the people I follow on blogs and twitter don't just broadcast cold ideas. But having said that, one of the mistakes I think people make when they first start blogging is to go too personal. I'm not particularly interested if you went to the cinema last night and the film was quite good and then you had a curry.
So It's about that bridging between personal and professional. Now if you went to the cinema, and had a revelation about open access while watching a 3-D slasher movie, I'm interested. I think the personal element is essential in good online presence (but that doesn't mean I want to know when you've had an argument with your spouse). Which raises the question, are there some roles where it's effectively impossible to do good social media? I think it's very difficult to be a police officer or politician or senior manager online as you end up having to sanitise every output to the point where it becomes unengaging. If you don't there are people who will use it against you. So, as you go higher up the career ladder maybe effective online presence becomes more difficult.