Facebook is social by design, is your idea too? No need to do it like Facebook of course, but very few b2c services can afford not to be social to an extent.
Indeed marketing expenses can be huge at the beginning, leading far too often to the death of the projects. Buzz happens it's true, not very often of course and it doesn't last.
Creating a social design, including all means to let users communicate within your service, enables your users to do your promotion outside of your immediate network. They can be very good at promoting you if they like you!