If Facebook were a country, it would be the 3rd largest in the world. The ranking would look like this:
Yep, it would beat out America. That is HUGE. That should put in perspective how many people actually use this social media site, and should indicate the importance of including it in your business marketing plan. The fact that the average Facebook user spends 55 minutes a day, or 6.5 hours a week, on the site should also clue you in.
The audience is already there, staring at the screen. All you have to do is utilize the Facebook Ads, Groups, Fan pages, etc, to position yourself in front of those who could benefit from your products and/or services. (And by utilize I mean actively engage your audience, not just set up a fan page and never look at it again).
I received these facts from a social media marketing webinar a few weeks ago, hosted by social media experts Lewis Howes and Sean Malarky. If you Google these guys, or even just "Facebook Marketing," you'll come up with tons of how-to articles and tips for using Facebook effectively for your business. Here is one to get you started: 14 Powerful Tips for Marketing on Facebook.
Morale of the story: If you're not using Facebook for your business, start right away!