I might be a bit naïve thinking this, and I only ask out of curiosity, but, OAuth non-browser implementation issues aside, what drove the decision to support four individual SDKs to access the Graph API as opposed to building the Graph API up into a resource that, like a well implemented REST API, could be used easily by anything that can make an HTTP request?

From that position, the open source community, or any enterprising developer for that matter, has a single, standard, and well supported API they can then write a language specific library for and maintain.

Like I say, I'm not privy to all the details behind this, and I'm being wilfully ignorant, but it intrigues me that the developer of an API feels that it needs to then support its own abstraction of it for a limited number of platforms/languages.