Facebook is quite simply huge. Over 300 million users now have signed up for the service and it's something you do not want to ignore. One of the great things about Facebook is if you already have a Facebook page (and who doesn't?!) then its very simply to setup a fan or business page from your own personal facebook account.
That way you easily administer your business page and keep a tab on who is a fan of it. of course the main downside to having a Facebook fan page is that fact Facebook is a closed system. Google does not index facebook pages (although it does index some basic information) so as a searchable marketing tool it isn't that great.
But where else can you get hundreds of thousands of fans to interact in a way that Twitter will not do. It's a great way to do Facebook marketing for your business.
Facebook also allows your fans to directly interact with each other which is fantastic... it's like a normal web forum in that regard.
Plus Facebook has a great arrow in its bag... Facebook advertising. For a small amount of money you can directly target your fan page to other facebook users and get them to like your product.
You can also get peoples email addresses and information when they "like" your page and give them special discounts and coupons etc.