Why do businesses have a Facebook page? A Linked-In account? A Twitter account?
Social networking sites in general have become some of the key online areas where friends can tell other friends about products, organizations, artists and world events.
Yes, marketing on Facebook etc. can have a viral effect – information spreads through networks exponentially... because if one person "likes" your Facebook page, that 'like' gets re-transmitted to all of *their* friends. Just don't lessen the importance of your main website in the process.
There are two primary ways for companies to market on Facebook. The first is through Facebook Groups and the second is through Facebook Pages. Groups are more focused on bringing together groups of people who share common interests, whereas pages are places where users can interact and discuss a particular brand or product.