Great question. My humble opinion is that sharing really only helps or use visibility. You have to consider the web and mobile web especially to be a sequence of steps. If they share, then what happens?

What do hey get out of sharing? Does the sharing benefit you or them? How can they tell there is value in sharing and if there is value, how do they experience it?

LinkedIn is a true example of a referral system that works by network effect. It's very clear for a user to see how she benefits from referring people to use it. More people for her to connect to. More opportunities for a career benefit. In fact, the incentivizing is built right into the media.

I'm happy to talk about this more. Let me know how I can help. Just remember that customers need two really important things. They need a reason to change current behavior and they need an example to show how changed behavior benefits them. Every new app or business on the web is an opportunity to choose.