You can already have a well established Ecommerce system, but Drupal can be used to set up a sub domain to host news, blog or a forums about your products. Or you can set up an application to create a competition in Faceboock or pull your latest twitts from Twitter.
Develop a Facebook application with the core Facebook Platform technologies: Social Plugins, Graph API and Platform Dialogs. Let your website interact with the Facebook social network: News Feed, Notifications, competitions and implemet any kind of viral marketing campaigns.