Today I learned:

1. "True viral growth is very rare; for a consumer internet product, a sustainable viral factor of 0.15 to 0.25 is good, 0.4 is great, and around 0.7 is outstanding."

2. "When the viral factor is less than one, it is crucial to have strong sustainable non-viral channels."

3. "Small increases in the viral factor can cause large increases in the amplification factor."

Bottom line? It's not easy to grow a user base, or everyone would be able to do it.

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