Andrew Chen’s blog Futuristic Play is one of the best sources for thoughts on viral marketing, startup metrics, and design: http://andrewchenblog.com/

I redefine it so that it’s not one singular effect, but rather, three distinct, underlying forces: the Acquisition Effect, which lets products tap into the network to drive low-cost, highly efficient user acquisition via viral growth; the Engagement Effect, which increases interaction between users as networks fill in; and finally, the Economic Eff