Know your audience?

So here is a question I've been contemplating overnight - who-all do you tell that you've started a blog? Can I answer that in the black box state of not knowing things like 'what the hell am I planning to put out there?' 'Can I control my tendency to navel-gaze?' 'What offensive soap box might I pull myself up on?' 'Do I really want my siblings to know I'm on Match.com?'