Correct me if i'm wrong, but facebooks developer platform seemed to me to create a portal that makes many functions of a desktop OS redundant, ie. an alternative.
I believe they are trying to create an online one-stop shop for everything. By developing an online application suite, they are trying to create a system where everything you could need is available through logging into the site. In a way, this approach is just closing them in further.
Does that make sense? Sorry, terribly explained, a few too many beers on a friday night!