I think Facebook's entertainment slant comes from its origins and is self-reinforcing. Take a look at the chapter I submitted for the upcoming "Psychology of Facebook" book. IMHO, it addresses the questions you ask. http://docs.google.com/Doc?id=dctjh9vw_1hs29mrgb
Plus, here is the site for the book, that is coming out of the sequel to the famous Stanford Facebook class: http://www.psychologyoffacebook.com
Would love to hear you thoughts on the role of context.