I'm not so sure yet but Facebook have published a book to tell you how to do it - "Facebook Insider's Guide to Viral Marketing"
Here's a synopsis
"Many businesses, from leading global brands to favorite local bands, are enjoying tremendous impact using Facebook Pages for free viral marketing. Check out some key strategies from the most successful businesses on Pages:
1) Regularly adding engaging and useful content
2) Letting fans participate in the conversation
3) Expanding their distribution with Facebook Ads
We’ve collected some of these winning strategies—along with the nuts of bolts of how to create and manage a Page—into an Insider’s Guide to Viral Marketing"