When we conceived Treehouse, we didn't say "let's create something super innovative and blow the world away", we asked "what if you could see a simple stream of your friend's mobile photos". We then decided to make a prototype and test it with a group of < 50 people. The results were astounding. People used it feverishly, and we're seeing the same exact behavior from our new users since the app has been in the store. Our user activity is growing extremely rapidly and engagement is through the roof.
It's not about innovation, it's about taking what is so blatantly obvious and making it possible. It's about addressing what people want. There's no good way to easily share photos that you take with your phone with the people that matter to you in a more intimate context.
Twitter was also a subset of a single Facebook feature.