When designing a new service on the Internet, the goal of an entrepreneur is to become mainstream. That means they want their service to be used by everyone.
Now let's consider the new services that became mainstream after Facebook. The main names would be Twitter, Foursquare, Instagram, Angry Birds... The common factor that made these service become so popular is the fact that they were really easy to use. Actually they didn't change our old habits. They integrated on existing platforms. Therefore the barrier to use it everyday wasn't huge.
|The use of an API enables to have a product on different platforms|
Therefore, when creating a product, startups have to make sure it can be used easily on existing communication tools. Nowadays, when it comes to online communication, people use:
Therefore you have to make sure your product can be used via theses tools and not only via a web app. This is even more true, when you consider that in the future we'll have more and more connected objects.
When you think about it... Instagram, Twitter, Foursquare don't even require a web app to be used. That's a true asset. There are some simple services that are really great because they smoothly integrate in existing platforms (like Summify or AgoraApp).
On the other hand there have been some awesome product that failed because of that. Google Wave was great... but it wasn't compatible with any existing communication tool. That pretty much killed the project.
This said... there are counter-examples. Google and Facebook succeeded to be mass adopted by changing the way people used to communicate. They're really disruptive. It doesn't happen often, but now those two rule the Internet.