Contagious: Why Things Catch On

by Jonah Berger

That’s how word of mouth helps with targeting. Rather than sending books to everyone, the publishers got me, and others, to do the targeting for them. Just like a searchlight, each recipient of the double mailing would look through his or her personal social network, find the person that the book would be most relevant for, and pass it along.

Virality isn’t born, it’s made.