You don't need to be available. Ever.

You don't need a facebook account, Twitter, a blog... I read only some weeks ago an article about a man who writes his books with pen on paper, he doesn't have a computer, internet, facebook, twitter, blog or anything like this. He's a "good old-fashioned author". And it works for him.
If you find yourself in Facebook when you should be writing, in Twitter, in the internet surfing and reading interesting things, writing in your blog, you might be using your internet to procrastinate what you should REALLY be doing. Writing.
Some writers do tweet their life away.

Why professional writers need a blog - or not. 
Do writers need to network? Do crackers need cheese?
How Twitter helps freelance writers
How social networking helps freelance writers
Television and your future self