As a former psychology high school teacher I used to talk about the phenomena of reciprocity. Reciprocity is about responding to either a positive action with another positive action, or as its more know, treat someone bad and they will treat you the same way back. We play around with this as phenomena with our Framkalla-app. Dropbox whole business are built around you recommending a friend to get more free space. We always ask ourselves why you as a user would like or even more share stuff online?
Here is Once Upons Studios example with Framkalla App:
We used the open graph Facebook solution to give the new customers 20% off their first order if they are willing to share our app to their friends. Easy as that. First, we say its an offer. And later, straight to the message. We try to establish a relationship with the user. Later on we ask them in a friendly way to like our Facebook page to get photo tips and other great help to become a better photographer. And seriously? Who wouldn't share something you actually like if the actually got money for it?
We give you this if you give us something in return..