By joining Twitter, Facebook, and other sites online, you can generate fans. Of course, you can't just put a profile online and hope to get people to buy your books. You need to interact with others in those sites and make friends. By making friends who aren't writers, you will get friends who idolize you and think its awesome that you've not only written a book but published one too.
And after getting to know you, they will buy your book just to get inside your head lol. I'm not saying that it will happen for sure, you've got to make it happen. Intrigue people. Make them interested in your words, your ideas. Some will go further than others. Share excerpts, reviews, give them a sneak peak to the characters. Through time, if you've worked hard, it will pay off. You just have to keep working at it and don't give up!
Even writers do read. So keep that in mind lol. Join as many networks as you can and monitor them, keep active in them, and keep writing!
You can't publish a book and then sit back and wait for them to be bought. You need to get your name out there and your titles out there in order to be found out. Even if you have a full time job, you still need to work at being known. But the rewards of it is endless and does keep you going, let me tell you. Hearing the words of praise is better than that paycheck from the book--it really is. For me, that is enough lol.