Platform APIs (e.g. Facebook, force.com, Google, etc.) are a great way to “let a thousand flowers bloom” and get the market to do the hard work of researching and market-testing new features. It’s an absolutely brilliant strategy. We’ll see the platforms acquiring their leading apps. We’ll also see (I bet) some nasty dust-ups where the platform copies features and competes directly with third-party apps.
However, developers beware — the fine print of many API agreements (such as Google’s) require you to indemnify the platform and/or agree not to sue for IP claims (e.g. patent infringement) relating to your work. This can make it hard to defend your ideas if the platform rips off your app, which in turn will make it hard to negotiate a good price if the platform wants to acquire you or license your app. Consult your attorney.