A few months ago, Shaun McBride, a prolific and popular Snapchat user, went to Bangkok courtesy of Marriott. He let his fans dictate his agenda, sending collages of visual messages, or snaps, at each tourist stop. "At the end of the day," he said, "I'd give a shout-out to Marriott for hooking me up with the hotels."

That kind of brand marketing thrives on the platform, explained the 27-year old, who was commissioned for similar work by Disney and has worked for AT&T and Samsung. To demonstrate what he won't do on Snapchat, he adopts a salesman patois: "Ten dollars off at your next stay!"

Brands must be hands-off, giving social-media savants like him one brief: "be true to yourself."

This was the overarching message from Mr. McBride and a trio of even younger players gathered on Wednesday by 360i, the Dentsu Aegis digital agency, for a panel on "Gen Z Influencers." The agency roughly defines the generation as those born between 1997 and 2002, and while the influencers in question might not be in the generation, they're definitely reaching them.

And marketers want to reach them, too, which is why they are increasingly turning to content creators with fame on mobile platforms such as Snapchat, Instagram and Vine. And they're shelling hefty fees to do so -- sometimes as high as five figures per snap, photo or video. The market's potential became clearer two weeks ago, when Twitter agreed to buy Niche, a digital talent agency for social influencers.

It makes sense. The influencers, like the YouTube stars before them, understand the platforms. And they can often execute two of the most desirable, difficult tasks for advertisers targeting younger audiences: mobile and native.

With his off-kilter images, Mr. McBride, who tucks his stringy, long hair in a backwards cap and cultivates a surfer dude image, has amassed a huge following of over 350,000 Snapchat "friends" known as "Shonduras." His most-engaged fans, he says, are often "14-year old girls."

Joining him on stage was another Snapchat celebrity, Christine Mi, 21, whose repertoire is blithe depictions of herself as historical figures or paintings. She has worked with ABC Family, 20th Century Fox and AT&T using Snapchat's Stories feature, when she is not studying at Yale. "I want the brand to become an enabler," she said, noting that those she has worked with gave her flexibility on the platform.

Amanda Jas, a 22-year old fashion photographer (over 78,000 followers on Instagram), has worked with Adidas and Refinery 29. Another Instagram star, Ryan Parrilla (83,500 followers), a shy 16-year old, just began taking brand work. He has an agent.

Working with young talent is not without its frustrations. Parents, of course, must be involved. And corralling the creative can be difficult -- they have a football game or they sleep in on Saturdays. And there's always the risk they have a mishap that could damage the brand.

Still, agency partners involved deem it worthwhile. "We're seeing crazy engagement rates," said Rebecca McCuiston, senior VP-influencer marketing, 360i, the host of Wednesday's panel.

Vikrant Batra, VP-worldwide marketing for Hewlett-Packard, a brand that has embraced influencer marketing, agrees. The company's strategy has been to cede creative control to its collaborators. He said: "Here's our brief: You guys have 6 million followers, because of the work you do, not because of the work I tell you to do."

Since May, HP has worked with its agency, 180LA, and Niche to find influencers, largely around its "bend the rules" campaign. Twitter has said little about how it will integrate Niche. (The company declined to comment, citing the infancy of the acquisition.) One agency executive said Twitter is sending pairs to meet with marketers, one of its own staffers and one from Niche, to shape content and paid media plans.

For Mr. Batra, Niche's strength is its software, a sizable database of thousands of social media eminences. "The dashboard for us, on a 24/7 basis, is incredibly helpful," he said. "That is the foundation of everything."

Niche works with influencers across multiple platforms. Yet Vine, its central tool -- and the one Twitter, its owner, hopes will net ad dollars -- was not mentioned on Wednesday night.

Perhaps it is getting too pricey. Some influencers charge around $2,000 per video; colossal creators, such as Logan Paul, the Adam Sandler of the six-second loop, can rake in as much as six figures, according to one agency executive. Or passe, like some other social-media platforms. The young celebs had little regard for Facebook. Yahoo's Tumblr did not fare well either. "I posted a picture on there once," McBride quipped.

But the Snapchat star was aware of a central marketing hurdle for his preferred platform, whose lightning-fast content offers little of the analytics now crystallized with social media marketing. So aware, in fact, that he serves almost as a spokesman for the company, now valued at an astronomical $19 billion. Snapchat "is more about the experience," he said -- a medium that can generate buzz and brand equity in ways beyond immediate engagement and sales numbers.

"Brands that get it are going to crush it," he added. "Brands that are stuck on analytics are going to keep tweeting over and over again."