Some companies try and recruit evangelists (loyal customers who help market the company), but I think a wiser route is to identify them by their actions, the way Pepsi does.
RetailEmail Blog: Takeaways from the Email Evolution Conference. 2008-Feb-14, by Chad White:
We identify email forwarders and treat them differently, rewarding them, said Lawrence DiCapua, Pepsi’s senior marketing manager. They reward them by promoting them to a VIP program within their Pepsi Extras loyalty program, along with others who show acts of evangelism. People have to requalify for VIP with each campaign—except for bloggers, some of whom are permanently VIPed.