In October 2015, thriller novelist Ruth Ware checked her social media accounts and discovered that Reese Witherspoon had posted a glowing 300-character review of Ware’s thriller, In a Dark, Dark Wood, on Instagram. (The book was released by Simon & Schuster’s Scout Press imprint that year.) “When I read this page-turning book about a bachelorette party gone wrong, I almost bit all my fingernails off!” wrote the social-media savvy star of such films as Legally Blonde, Walk the Line, and Wild, earning 54,057 likes. The post included a photograph of the novel framed between impeccably arranged gourds.

“I’ve had so many readers tell me that they picked up the book specifically on her recommendation,” Ware said. “That’s part of the charm—you get readers who might never have chosen your book picking it up because they’ve loved Reese’s other picks.”

Witherspoon’s publishing influence began in 2013 with her Instagram endorsement of The Engagements by J. Courtney Sullivan. The actress currently counts 10.8 million followers who collect her bite-size book reviews, marked with the #RWBookClub hashtag, on both Twitter and Instagram. Unlike most book club leaders, Witherspoon sometimes selects books for more than reading—her production company has optioned both The Engagements and In a Dark, Dark Wood.

Since Witherspoon began her informal club, a number of other celebrities have followed suit, sharing literary recommendations on a variety of social media platforms. Instagram remains the dominant outlet, and these new influencers are popularly referred to as bookstagrammers.

While this movement began with spontaneous expressions of love for particular titles, it has become a serious part of every publicist’s media outreach. “We really value what bookstagrammers are doing,” said Meagan Harris, who worked on Ware’s campaign as the publicity manager at Scout Press.

Witherspoon has now recommended three books by Ware. According to the novelist, the most recent endorsement came after “some behind-the-scenes messaging” between the S&S publicity team and Witherspoon’s team. “I’d say the majority of our contacts are generally established media outlets,” Harris said. “But Instagram influencers are a big part of our publicity list and we work with them just as we’d work with more traditional media.”

Other platforms have also provided fertile ground for book clubs. Actress Emma Watson founded the feminist book club Our Shared Shelf at Goodreads in January 2016. Her first pick was Gloria Steinem’s 2015 memoir, My Life on the Road. The Our Shared Shelf community of readers has grown to more than 204,000 members. This spring, actress Kim Williams-Paisley launched a book club on Facebook and Instagram. In July, she recommended E.B. White’s novel Stuart Little to her 137,000 followers.

In March, actress Emma Roberts and producer Karah Preiss launched Belletrist, an online book club with a website, Tumblr blog, email newsletter, and Instagram page, that invites members to “discover, read, and celebrate a new book every month.” Megan Fishmann, associate publisher and director of publicity for Counterpoint Press and Soft Skull Press, noticed that a Belletrist post had mentioned the work of Eve Babitz. Counterpoint was about to reissue Sex & Rage—a 1979 novel by the Los Angeles author—so Fishmann reached out to the book club. In July, Belletrist featured the book across all its platforms.

Babitz found the whole experience “thrilling,” finding readers of a new generation and through new channels. “The response has been astounding. Readers meeting [Sex & Rage protagonist] Jacaranda and 1970s Los Angeles in 2017 and having such a positive reaction to her means everything to me,” Babitz said.

Oprah Winfrey is the patron saint of these 21st-century book clubs. Between 1996 and 2010, Winfrey led her own book club on the Oprah Winfrey Show on ABC, and a Winfrey selection was a ticket to the bestseller list. Winfrey still runs Oprah’s Book Club 2.0, the digital incarnation of her old book club; these literary conversations now happen on the smaller outlets of Winfrey’s OWN and online and pack a smaller punch than her original recommendations.

Social media wizards can’t make the same publishing waves that influencers once produced in a less crowded media landscape. With the proliferation of platforms, the overall reach of any single voice gets diluted. “It’s going to be harder and harder though to see the kind of bumps that we used to see from a big celebrity endorsement,” said Kristen McLean, executive director of business development at NPD Book. “I don’t think we’ll ever see a pattern like we saw with Oprah, just because the whole discovery mechanism has now become so diffuse.”

Even though she didn’t see a notable sales bump from Witherspoon’s book recommendations, McLean placed value on “any help” a book can find in the contemporary media scene. “Certainly, discoverability and getting advocates for a book in the current environment is super difficult,” she said. “Somebody advocating for a book—on any level—in the public sphere is great for the book and great for the author.”