To see metrics on your Facebook Page or Platform application, go to the Insights Dashboard. Only Page administrators, application owners, and domain administrators can view Insights data for the properties they own or administer. To view comprehensive Insights on your specific Page, Platform application or website, click on the corresponding item on the left navigation bar.
Insights is a free service for all Facebook Pages and Facebook Platform application and websites. In addition to the Insights Dashboard, data is also available through the Graph API. To learn more about the Facebook Platform and Graph API, visit the Platform and Developer Support help pages.