Thanks for the link, Chris. Very informative.
An interesting idea about the exclusivity too. Yes, that can certainly stimulate sharing.
Unfortunately, this can work only for a short while, because as the virality kicks in, the exclusivity will disappear by definition. But it can be a good way to seed the virality in the first place.
I can add another similar method - personal favors. For example, if a close friend of mine asks me to share his new product, I'll probably do it (and vice versa) regardless of the product itself. Again, this is good only for seeding, because the number of people who can do me such a favor is limited.