It is all depending upon you. Do you want to take the time to do it?
Fortunately, there are sites that will update all of it for you. For example, on Facebook, I use RSS Graffiti , it will post my blog posts on my Facebook for me, I think it may also post it to Twitter, I cannot remember how I have it set up. I have over 500 followers on twitter, so I make it a point to be active on there a few days a week. On Facebook, I have my personal page, but I also have a page that i manage that is my blog. I try to get on that at least once a day to interact with people (although, as of yet there aren't any interactions).
Most popular blogs keep up on both sites. But really, it is your choice.