A book by Billie Eilish seemed like a great bet. One of the most famous pop stars in the world, Ms. Eilish has 97 million followers on Instagram and another 6 million on Twitter. If just a fraction of them bought her book, it would be a hit. But her self-titled book has sold about 64,000 hardcover copies since it came out in May, according to NPD BookScan, which tracks most printed books sold in the United States — not necessarily a disappointing number, unless Ms. Eilish got a big advance. Which, of course, she did. The book cost her publisher well over $1 million. 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. Even having one of the biggest social-media followings in the world is not a guarantee. “The only reliable part about it,” said Shannon DeVito, director of books at Barnes & Noble, “is that it’s unreliable.” An author’s platform has long been something publishers look at — does she have a radio show, for example, or a regular guest spot on TV? But as local news outlets and book coverage have dwindled, the avenues for book publicity have shrunk, making an author’s ability to help get the word out more crucial. And when an author speaks to her followers about a book she wrote, she is talking to people who are at least a little bit interested in what she has to share.