The millions of people who follow Kim Kardashian West and her sisters on social media have become accustomed to seeing them praise everything from fat-burning tea to gummy vitamins for healthier hair.

“Ever since I started taking two @sugarbearhair a day, my hair has been fuller and stronger than ever!! Even with all the heat and bleaching I do to it!” Khloé Kardashian posted on Instagram this month.

But in the last week, close watchers of the sisters’ accounts may have noticed a small addition to those laudatory messages about the latest miracle product: “#ad.”

For marketers contending with consumers who use ad blockers online and have cut the cord to their TVs in favor of streaming services, social media has become a way to reach an elusive audience. Brands such as Jack in the Box and Red Bull have proved willing to pay thousands of dollars per social media mention to people like the Kardashians and other so-called influencers who command big, loyal followings on services like Instagram, Snapchat and YouTube.