Promoted posts are something relatively new (as of the writing of this book), but also potentially explosive for businesses looking to use Facebook for marketing purposes. For each post you leave on you your fan page, you'll have the option to promote it so that more people will potentially see it. Simply click the Promote button under any post that you put up, choose how much you want to spend (higher spending levels will have higher reach), and decide who you want to see the post (people who like your page, or people who like your page and their friends), and your post will show up in the newsfeed of potentially thousands of people. (This works better the more fans you have.)
While Facebook is the largest social network, it's not the only one. In fact, there are tens, if not hundreds, of thousands of networks that do all sorts of different things and reach a variety of audiences that you may also want to consider. I'll just go over a couple here, but popular other networks include MySpace (yes, it's still relevant), Yelp, Digg, Reddit, Zvents, YouTube—the list goes on. Any one of them may be able to help you get your site seen by your target market.