The problem with both Facebook and Twitter is that they haven’t completely figured out the role of 3rd party developers. They may say they embrace the development community, but they don’t really respect the community. Companies like Salesforce and Google have figured this out, mostly because they are committed to helping 3rd party developers make money as a way for themselves to make money.
Sometimes it takes a lot less effort to validate assumptions about your product-market fit than one might think. You can come by customer feedback earlier and with less effort using Lean Startup methodologies.