The more I read Facebook's statement about opening its platform to third-party developers, the more it seems like you could interpret what they say as a promise to open just their APIs, or both their APIs and their underlying platform code. Which one's more likely? Better to ask: which one makes the most sense for Facebook, or any other Web company?
I'm betting in this case that it's only a set of APIs, or at most APIs plus the code for a toolset that isn't the core of Facebook itself. But that's not a moral pronunciamento; it's just a reflection of what Facebook is. Companies like Facebook -- or Google, or Amazon -- make their living by selling a product or reselling some part of their service in a controlled way. Providing APIs for their services makes more business sense to them than providing their platform code.
There's no reason Facebook can't do both, though; it's largely a question of scope and what they want to accomplish with it. One thing I'm sure they don't want to accomplish, though, is to make it possible to no longer have to rely directly on Facebook to get what they offer. As long as their business model involves being unique as a service, they'll do what it takes to remain unique.
Contrast this with a company providing a wholly open platform, code and APIs alike (easy example: MySQL). They're going to profit by dint of offering something else that's unique. Typically, it's managed support or other direct-sales expertise -- in other words, they're still reselling the brains behind the outfit, just in a different package.
So which one's better, and not just in the sense of which is better for my immediate needs? For many people, an open API to a relatively insular platform is going to be "open enough" for them. But if you'd rather not depend wholly on any one outfit to supply a set of APIs, then an API set alone won't do. I'm curious to see which way the majority of Facebook users opt to go.