What you could do if you were Google and had their databases
I recently attended a talk at Cornell by a guy who is due to start working at Google soon.  He had not started working there, but presumably he was hired on the basis of the work he described, and he was explicitly interested in applying his framework to the Google social network.He was working on a scheme to get people to recommend things to their friends in return for discounts on those things.  His central focus was on devising a payout scheme in which people's economically optimal strategy is honestly reporting the cost of making the recommendation, and then choosing the optimum set of people with whom to make this deal for a given budget.  Optimum in the sense of influencing the most people with the recommendations.