Moontoast has done an incredible job on developing a visual representation of the anatomy of Facebook engagement. They rank fans in the Fan Engagement Spectrum from potential fans to super fans, provide the data elements necessary to gauge success, as well as the paths that each type of fan takes to generate a positive return on your investment.
From their post on Facebook Fan Engagement:
At the highest level fan count (Liking a page) is not a good metric for engagement. Community engagement (likes and comments) on average sits around 3%. To increase interaction with the fan, utilize different types of content to drive conversations that deepen the relationship with the community and drive potential fans to superfans. Introducing commerce as part of your overall social media strategy is a fantastic way to reward fans and create buzz about your Facebook page. This all requires an understanding that the world we live in today makes it easier than ever to have real relationships with global customers and to increase brand affinity. Treat customers well and word will spread.