Millions of Followers? For Book Sales, ‘It’s Unreliable.’

It's difficult to predict whether a book will be a hit. A jar of tomato sauce doesn't change that much from year to year, making demand reasonably predictable. But every book is different, an individual work of art or culture, so when the publishing industry tries to forecast demand for new titles, it is, however thoughtfully, guessing. Because there are so few reliable metrics to look at, social-media followings have become some of the main data points publishers use to try to make their guesses more educated.

An author's following has become a standard part of the equation when publishers are deciding whether to acquire a book. Followings can affect who gets a book deal and how big an advance that author is paid, especially when it comes to nonfiction. But despite their importance, they are increasingly seen as unpredictable gauges of how well a book is actually going to sell.