What you are building is either an app or a platform. If you try to make it both you will fail. And there are very few (if any) examples of successful platforms that started as a platform.
Point 1: If a startup tries to be both an "app" and a "platform" it will fail.
In my experience, when building a new product, teams need to make a choice very early on whether they are building something that is all about end-user value/experience ("an app") or all about 3rd party value ("a platform").
Groups that attempt to do both lack focus. They don't do a great job building a great user experience and they don't do a great job building the platform because these two things are in tension. Example: enabling a 3rd party plug-in model requires the creation of UI that is often complex from a user's perspective. The investment in that UI to make it great (especially at an early stage when you don't really know what or how many 3rd party extensions will get built) could be better spent on building end-user-valuable functionality in the core of the product.
This point is all about focus. To be successful startups need extreme focus.
Point 2: There are very few (if any) examples of successful platforms that started as a pure platform.
Name a few successful "platforms". Before you do, for the sake of argument, agree with me that "successful platform" implies the platform has a vibrant set of independent partners who both contribute to and "profit" from the platform. I could also use the term "virtuous platfrom" which has come to mean a platform composed of a multi-sided market where each side of the market derives value in a symbiotic cycle with the others.
[EDIT added link] http://en.wikipedia.org/wiki/Virtuous_circle_and_vicious_cir...
The "virtuous platform cycle" that most people use as the prototypical example is the Windows ecosystem. The market sides were: Windows, Intel, OEMs (e.g. Compaq, DELL), IHVs (e.g. ATI, HP printers), ISVs (e.g. Lotus, Adobe, Office), retailers & channel (e.g. Egghead), and of course, end users.
All of these "market sides" drew great value from one another as part of the Windows ecosystem. They all made money, hand-over-foot, because the "virtious platform cycle" snowballed.
There are many examples of other platforms with a virtuous cycle today. Google's search engine, Apple's iOS, and on a smaller scale something like Minecraft (although that's a nascent example).
For each example YOU come up with, ask this question: What came first, "end user value" or "developer/3rd party platform"? In my case, I struggle to find examples where the platform STARTED as something that was focused on developer/3rd party value. You might argue Java, but then I'd say "in what way did the creator of that platform make any money off the platform?".
Discuss. :-)
Both are Apps, and both allow you to write code against them (or maybe Twitter doesn't count because it doesn't run on their site?)
Facebook did not add an API until much later and "being a platform" did not become a priority until recently.
But that's history. These days, with the Internet for infrastructure, creating user-facing platforms and use of social-software provisions blurs everything, with an increase in user-facing provisions (and users for addition of value), the process is short-circuited, sometimes massively. But today's attention to ecosystems and the effort to establish Windows Phone as one platform of an ecosystem is further example.
Also, I think there are other reasons for apps to be modularized in a way that supports plug-in extension and customization in larger integrations. Attention to extensibility and customization can be valuable in the development progression of the app itself. That might make it a mini-platform without growing to a platform as we think of them in todays distributed ecosystems. It is useful to have an eye on such matters so that early releases can grow there from initial rudimentary (essentially-private) provisions.
Further, most startups that try to build a platform from the beginning fail, because to have a platform you need a vibrant set of partners who make money off your platform, and it's very rare for a startup to be something that others will build a business on.
Perhaps those that build a marketplace could be viewed as both an app and a platform, like an Ebay. Of course it's necessary to get scale as the marketplace for it to really be successful and provide value to both sides of the transaction (buyers and sellers).
Note that the Amazon example shows how Amazon transformed from an app to a platform by adding the marketplace.