There are many photo editing apps and many offer some kind of sharing.
Yet Instagram (see my previous post) is way ahead of the pack in popularity and buzz. This fantastic article goes into why. My favourite aspect is about how it deliberately limits its functionality. you can only load one image at a time. Imagine how natural it would seem to add many at once. But that would detract from the product’s simplicity and delight.
Design is finished not when there’s nothing left to add, but when there’s nothing left to take away.