For many marketers, their Facebook fan bases have become their largest web presence, outstripping brand sites or e-mail programs either because a brand's traditional web-based "owned media" is atrophying or because more consumers are migrating to social media.
In my opinion wrong question: Facebook OR corporate web site. Companies need a supplemental strategy integrating and combining the Facebook Fan Page and the Corporate web site. And they need to go away from the corporate web site as colorful shop-window and proceed to a web site offering dialog and communication with integration of social media for their customers. Companies need to provide an exceptional web experience and value on their site. This is the way to go.
P.S. And their is obviously a difference between B2C and B2B.