As someone who's tried, too many times, to build a platform first product strategy (leveraging 3rd parties for distribution with a white label infrastructure play) I strongly suggest you DON'T.
Unless you're building extremely primitive services (think Twillio, AWS etc) you should instead build a killer user-facing (consumer or business user) app (hard!), and then offer APIs, Widgets etc for 3rd parties to adopt once they have no choice but to play along.
Think FB Social Plugins, YouTube Player, Twitter Tweet Embeds etc.