If you are a tweeter definitely use Twitter. Especially if your target market is between the ages of 18 and 49. Twitter can be amazing for marketing. It is the best real-time social media tool out there. There are some markets that twitter works especially well in, and the literary word is one of them. Probably because writers think they always have something to say :)
If you aren't sure about Twitter, then check it out first. There are tricks to using Twitter successfully. You really need to have a smartphone. And be willing to be on it whenever you get a tweet. I, personally, have used Twitter with clients - I know how it works. But for myself, I'm not quite ready to commit. And I don't know a lot about tweeting as an up-and-coming author yet. I have to learn the market. So, since I have the time because I don't have a book to promote yet (or even an agent!) I am easing into Twitterdom with...
Step 4: Stalking
In other words, I have created a Twitter account (laabwriter). I am searching out who I want to follow (mostly other writers, agents, and publishers). I am not following friends - well a couple. I am not following anyone who is not in the market or business I am in. I haven't tweeted at all (except for a greeting to a friend) and before I tweet a single word, I am stalking all these others who know what they are doing. I'm not kidding. You learn by watching and carefully observing. Watching other writers use Twitter effectively to market themselves is excellent information. I am stalking and learning. Once I feel that I have a sense of what seems to be successful for writers, I will use Twitter more fully. But for now, beware - I'm watching you!