On the Victoria’s Secret website, you can search for the names of its representatives at 100 campuses, in schools from Columbia University to Grand Valley State University.

At Virginia Tech, as many as 1,000 of the 30,000 undergrads are being paid to promote products as varied as mascara and storage bins, according to an estimate by Donna Wertalik, director of marketing for the university’s Pamplin College of Business.

“We see so many brands that have it,” Ms. Wertalik said. “A lot of start-up brands will do it. They’ll look for students with credibility and influence to give them credibility and influence.”

Ms. Wertalik oversees a student-run ad agency called Prism. Of the 45 undergrads employed there, she estimated, half are paid to promote products on Instagram.

Companies outline expectations for what the sponsored posts should include, such as specific hashtags or promotions for particular items. Many also ask students to hold or attend events on campus.

Isabel Senior, a student at Duke University, worked for LaCroix, the sparkling water company. Each week for six weeks, she had to post one Instagram photo, one Instagram story and one post on a platform like Facebook or Snapchat. She also gave out cans of LaCroix at campus events like five-kilometer races and university-sponsored concerts.

Every Sunday, Ms. Senior had to send in a form with three photos from her weekly sampling event, along with screenshots of her posts. The company paid her in money and LaCroix products, and if she didn’t complete a task, she said, it docked some of the pay.