When Facebook introduced its plan to be a platform for developers to work on, I loved the idea. It reminded me of Yegge's post on Amazon, and seemed to me like a great way for Facebook to evolve to be the 'social layer' of the web.
Of course, they torpedoed that whole effort and kept going with their walled-garden facebook-as-a-product approach.
And now that Facebook seems to lose some momentum/popularity (or is perceived to be, which is still bad), they end up implementing part of the 'platform model' anyways, except still within their walled-garden with facebook-made apps.
It's quite possible that I'm missing the real reason why they are taking this approach, but seeing Zuckerberg as a product-first guy might be a partial explanation.