Ask HN: Product first and API later or API first and product on top of it

In scenarios where a product and its api are equally important what is the best way to go forward? 1) Focus on product and build api later. 2) Build an API first and product on top of it, using that API Also, I wonder if social networks like Google+, Facebook or FourSquare use their own API. Do they have separate codebase for API and Application?