Reason number 3 for best sellers—and most people would say this is number 1—is what we call word-of-mouth advertising. The Road Less Traveled came out in the mid-1980s and stayed on the New York Times' best-seller lists for 12 years.
Some readers liked the book and told others who bought it. They liked it and passed on the word. The Shack is another word-of-mouth phenomenon, and that one started as a self-published book.
Many people would put Twitter, LinkedIn, Facebook, and other social media sites here as one form of word-of-mouth advertising. To my knowledge, there's no strong evidence to support sales based on posting to 7,000 friends that you read and liked a book.
I buy most of my books from the recommendations of friends whose tastes are somewhat like mine—that's the idea behind word of mouth. If someone whose tastes I respect raves about a book, I check it out. Many times I buy it immediately; other times I read the reviews before deciding.
Jonah Berger's Contagious: Why Things Catch On stresses word of mouth and calls it "the primary factor behind 20 percent to 50 percent of all purchasing decisions." He goes on to say that "A five-star review on Amazon.com leads to approximately twenty more books sold than a one-star review."
I quote Berger because he can back up his data and he makes a significant statement about word of mouth: ". . . is naturally directed toward an interested audience.”
 Contagious: Why Things Catch On by Jonah Berger (Simon & Schuster, 2013), p 7.