I love the part in the Facebook movie where the product hits exponential growth a few hours after it was released:
“Harvard at that time did not have a student directory with photos and basic information, and with the initial site generated 450 visitors and 22,000 photo-views in its first four hours online.”
All of the features, designs, and A/B testing in the world wouldn’t have achieved that level of adoption on their own.
To me, this growth shows that Facebook clearly met some quite fundamental human needs that had been unfulfilled on the web until that time – perhaps the need for self promotion and some re-enforcing of social status and groups by sharing a laugh at some of the early photos with your friends.
As features were added that gave people the ability to express themselves, form friendships and groups, and compare themselves with their peers, people obviously continued to adopt the product extremely quickly all across the world and across cultures. Again, this was for quite human rather than technical reasons.
If you’re looking for your product to spread or go viral, I believe that you need to appeal to people on a similarily primal level. Think about how your product can tap in to peoples low level needs, desires, fears, and wants. If you make the user genuinely feel better or more successful after using the product then they’re much more likely to return and spread the word.