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.