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Media companies like Viacom and Tumblr pair brands with social media sensations like Todrick Hall, a YouTube star.Credit Jake Michaels for The New York Times

Ricky Dillon, 23, who is known for creating quirky online videos, has millions of followers on Instagram and Twitter. His YouTube channel has more than 2.5 million subscribers.

When he posted a picture on Instagram of two Coca-Cola cans — one with the name Ricky on it, the other with Dillon — many of his followers were ecstatic.

“Awesome!!!!!!” one user wrote. Another urged him to sell the cans on eBay.

But a handful of viewers were less enthusiastic.

“Was this a paid sponsor?” asked a user going by the name MikeVlogsYT. “Haha.”

Indeed, it was: Mr. Dillon had promoted Coke as part of an ad campaign for MTV’s Fandom Awards.

As advertisers struggle to connect with young audiences, many have turned to so-called influencers like Mr. Dillon: video and social media stars whose value essentially lies in the large numbers of their followers. Such influencers offer brands the ability to amplify their messages at a relatively low cost.

But the strategy is becoming a bit of a gamble. The more brands that use influencers for marketing campaigns on social platforms like YouTube, Twitter and Instagram, the less impact each influencer has. At the same time, many influencers, who once jumped at the opportunity to endorse brands, are being much more selective for fear of appearing to sell out.

“It was too easy money for a lot of people,” said Tony Weisman, the chief executive of the digital agency DigitasLBi North America. “We feel that chasing the influencer has in fact generated people that aren’t too influential.”

Advertisers say they work with influencers to build brand credibility on social media and to turn promotions into more of a person-to-person conversation. And with consumers increasingly viewing — and sharing — content online, the tactic has only become more popular. Media companies like Viacom and Tumblr have developed marketing platforms that connect brands with influencers. At least one advertising agency is putting influencers directly on its payroll.

Most influencers are young, with a cool factor tied to the number of online “fans” they have. (The fans may or may not be real people — bots are prevalent.) Many do little more than talk into a camera or take eye-catching pictures.

Mr. Dillon, for instance, creates what he calls “fun videos” — mostly comedy skits and music videos — which he uploads to YouTube every week. In one popular video, titled “What’s in My Mouth,” Mr. Dillon is blindfolded and is asked to name the objects that are placed in his mouth, including a condom (“This is flavored”) and an eggplant (“That is huge”). The video has received more than four million views, 6,000 comments and nearly 300,000 likes.

Those numbers can represent a gold mine for advertisers, especially for fans like Mr. Dillon’s, who are young and active.

Data on advertisers’ use of social influencers is hard to come by. But according to a 2014 study from the content marketing company IZEA, 52 percent of marketers said they had used social influencers in the previous year. By comparison, 58 percent of marketers in the study said they had used display advertising.

Advertisers over all say engagement — generally measured in likes, comments and shares — is higher for content that comes from influencers.

AT&T said engagement per post on its Instagram feed had increased about 250 percent on average since the company teamed up with Dave Krugman last fall through the advertising agency BBDO. Mr. Krugman, a photographer who now has nearly 200,000 Instagram followers, was hired by BBDO last summer to help clients with social media campaigns.

John Osborn, the president and chief executive of BBDO New York, said hiring Mr. Krugman reflected the industry’s growing interest in social advertising.

“Social isn’t just an add-on anymore,” he said. “Social is really core to all ideas, and the way to build credibility through social is through authenticity and through influence.”

As brands rush to hire influencers, companies have also started offering services that connect the two. Viacom, which linked Coke with Mr. Dillon, has built a “social talent platform” to help advertisers identify influencers for specific campaigns. Recently, it also connected Todrick Hall, a star on YouTube, with Fiat for a commercial that cross-promoted a new Fiat model and the MTV Movie Awards in April.

The blogging platform Tumblr has also taken advantage of the rising demand for influencers. This year, it unveiled an initiative called Creatrs, a network that connects artists on the site with brands. Among brands that have connected with artists through Tumblr are Unilever and Converse, which is owned by Nike.

Although many advertisers look to social superstars to act as cultural megaphones, others court influencers with smaller but highly engaged audiences. Beauty brands like Origins, an Estée Lauder company, have teamed up with lesser-known influencers for more targeted campaigns.

Origins worked with Plaid Social Labs, which has a database of roughly 22,000 influencers, for help in choosing brand advocates. Plaid Social Labs, which was founded in 2009, was acquired this year by the Corbis agency.

Ricky Ray Butler, a founder of Plaid Social Labs, said more brands than ever were coming to his company for similar middleman services. But companies’ rising interest in using influencers has also meant that influencers are getting smarter with their sponsorships.

“Early on, they would almost say yes to everything,” Mr. Butler said. “But now, they’re a lot more picky.”

Mr. Dillon, like other influencers, said he had become more selective than when he started working with brands. His marketing partnerships, he said, have to feel organic. He worries about losing his cachet.

“That is still a big concern for me,” Mr. Dillon said. “I pass down a lot of brand deals.”