I have a blog on Wordpress that I update every weekend (and inbetween if some news comes up, of course.) I've created a monthly schedule for it, so I have a particular type of entry for each weekend of the month, which as least gives me a head start on coming up with something to write about! I set myself a target at the start of the year to update it every weekend without fail and I've kept to that so far. (The "schedule a post for later" option, and being able to deploy posts from the WP app on my phone have been useful for keeping to that a couple of times!)
I've given in to the peer pressure and made a Page on Facebook for my pen name. My Wordpress blog automatically posts to it, and I post other things sometimes. I'm not that big a Facebook fan, but I'm getting more comfortable with it. I do prefer the Page rather than Profile for my author self, for people to Like rather than Friend. That just suits me more.
I've had a Live Journal account for years. It may be a bit old fashioned, but it's where I feel happiest. I use that a lot - though for some reason a little less than I used to. It's not for heavy "promotion" really, just sort of a "what am I up to and what am I thinking and wittering about right now" and to interact with other writers and friends.
Goodreads I joined back in 2009 and got an author profile in January this year when my first novel came out. I use it a lot, but still mostly as a reader. Like with Facebook it automagically picks up my Wordpress posts to be my author blog on Goodreads. I've never got too heavily into the groups on there, some of the likely ones would be a huge time suck for me.
I've marked my spot to Google+, but am not impressed with it and haven't done anything on it so far. LinkedIn doesn't really seem like it would be useful to me. I've got a Tumblr, but only just started to play with that. Not done much with it yet.
AW I'm on, obviously. And the NaNoWriMo boards, especially in NaNo season - like now!
Twitter is where I'm especially active. I love Twitter. I will post promotional tweets of course when there's some news, but mostly I'm just, well, twittering on. Sort of promoting myself - or at least my writing persona - as (I hope!) someone interesting, engaging, funny, approachable.
It's also a great place to collect interesting links, especially if you follow lots of writers, as I do. I save up those and other links in One Note and post a list of the best ones once a month on my Wordpress blog. So that gets me some continuing value out of what is otherwise "here now, gone in ten minutes" content on Twitter.
I've also set it up so that when I post to Live Journal, Wordpress or a Goodreads update it automatically tweets a link to that, to hopefully drive people from my Twitter feed to my sites with more permanent content on them.
Also, Twitter is just FUN. I was on a while it before I was published, though I ended up creating a new account for my pen name and leaving my old one behind. (I know, I could have just changed the name, I did with Live Journal, but a new account was just the way it worked out.
I think there's no set rules for which sites you have to use. And trying to use all of them just takes too much time and makes you start to feel like you're repeating yourself. It's better to find a couple of sites that you're actually comfortable on and use them, than try to use one you don't really get and have no enthusiasm for using.
If you do use more than one, I think it's good for have to be different purposes to each of them. My Wordpress blog is one where I do regular entries once a week, of fairly formal blog posts. Live Journal I update ever couple of days usually, with more personal stuff about how the writing is going, and other things on my mind. Facebook is (or at least heading that way) primarily news. Twitter I'll be on several times a day posting the usual nonsense people post on Twitter, and replying to other people's nonsense. So anyone following me on all of those shouldn't feel as if they are reading the same thing over and over. For example, I'd post on my Live Journal that I've put up a blog entry on Wordpress, but I wouldn't actually cross post the whole blog entry on Live Journal. I keep some separation between the different venues and what I do on each.