http://www.linkedin.com/today/post/article/20121002124206-18876785-how-to-model-viral-growth-the-hybrid-model

My take-away was that it’s a myth that there’s such a thing as virality coefficient > 1 (where propagation is self-sustaining) but that making something “more viral” is good and viable — it just means getting more bang for your marketing buck. Of course it will still immediately fizzle if you stop marketing.

The link above suggests that if you can get something like virality 0.7 (which the article calls “outstanding”) then that can triple the your non-viral user acquisition.

Or a simpler takeaway: No matter how viral you think you are you still have to do non-viral user acquisition. There has to be something for the virality to amplify. Because actual virus-like virality, ie, self-amplification, doesn’t really exist.

It sounds like Sharad’s data suggests that viral propagation is literally a one-in-a-million event (or even a bit rarer than that).