Jenna Marbles, watch your back. Your 18.8 million YouTube subscribers may still be rapt, following your adventures as a young millennial. But the influencer world can be fickle. Behind you are a legion of Gen Z wannabes just itching to take your place.

Already, the biggest YouTube creators face being elbowed aside by "younger influencers who are shinier versions of them," says Bing Chen, former head of creator development for YouTube and founder of Bingdom Studios, a content studio and strategic consultancy. As millennials get pushed aside by Gen Z influencers and their power gets diluted by numbers, Chen sees a shakeout coming in the influencer ecosystem.

"It's not that influencers are dead," he says. "But the heyday is close to over" because the amount of churn means current stars are likely to fade fast.

While previous generations tended to look up to older influencer groups because they were seeking people who were aspirational, Gen Z looks up to people their own age. And there are plenty of places to find those peers, says Clyde McKendrick, chief strategy officer at Sparks & Honey. A single member of Gen Z burns through 68 videos a day on YouTube, Snap and Instagram, according to a survey of 1,000 13- to 17-year-olds conducted by Awesomeness and Trendera this summer. The report also revealed that the majority (55 percent) of Gen Zers don't have to see someone in person in order to feel connected to them, which is why marketers are rushing to sign up influencers to court a generation expected to wield $44 billion in buying power by 2025, according to a Sparks & Honey Culture Forecast.

It is, of course, a strategy that could backfire. For every JoJo Siwa, there's a Logan Paul. For measurement-minded advertisers, the audience metrics are decidedly dodgy for a long tail of influencers that not only distribute their advertising, but create it. And there's the added risk of working with very young and potentially impetuous talent.

Case in point: CoverGirl selected James Charles, an 18-year-old high school student from New York, as its first male CoverGirl model, who was later called out for a tweet that appeared racist. Yet despite the risk, CoverGirl and many other conserverative marketers are embracing influencers. CoverGirl owner Coty even has a VP-influencer marketing, Cheryl Dixon, whose role encompasses dealing with beauty editors and other traditional media as well as paid influencers of all ages.

Clorox's Brita water filtration brand, which has leaned on more conventional star-athlete endorser Stephen Curry in recent years to pitch its products, recently added former Vine celebrities turned YouTube creators King Bach and Rudy Mancuso to its influencer stable. Mancuso has also pitched Axe from Unilever, which has tapped influencers to develop fragrance brands in Europe.

One reason is cost: Though mega-influencers like Mancuso aren't cheap, they often work for around $20 per thousand impressions based on their follower counts or estimated reach, which, considering they deliver both talent and distribution, can be a bargain relative to TV, according to people familiar with the matter. (CPMs for broadcast and cable TV nets range from $45 to $60 just for the media.)
Clorox's Brita already had Curry, but decided to supplement him by "adding another layer to our digital marketing efforts" last year with King Bach (1.7 million YouTube subscribers), says Associate Brand Director Yating Wong. The brand saw enough success there that it paired Curry more recently with Mancuso (3.9 million subscribers), described as "someone that truly resonates with our target and can inspire them to choose filtered water."

For all generations, social media stars appear to be better at getting attention for marketing messages than more conventional media. Research from Shareablee covering more than 100,000 brands finds that branded-content posts by influencers on Facebook, Instagram and Twitter generate more than 50 percent more engagement—likes, comments, shares, etc.—than content distributed other ways, including conventional paid ads through publishers.

Sharablee CEO Tania Yuki says influencers appear to combine aspects of star power along with greater "believability" for their fan bases. In fact, a detailed look at activity by more than 40 influencers found posts of branded content that were labeled as sponsored had no less engagement from viewers than posts that weren't compensated. People also tend to trust influencer posts more than those directly from brands, Yuki says. "By human nature, we trust people rather than corporations."

Bob Gilbreath, CEO of Ahalogy, last year struck a deal with Oracle's Moat to measure audiences and engagement for influencer campaigns his firm works with. Videos created by Ahalogy influencers averaged 7.5 seconds of viewership, which, while it may not sound great, is more than triple the level of under two seconds for brand content in digital media generally.

Ahalogy, however, is a relatively rare player that subjects influencer campaigns to third-party audience verification. While it's possible to get verified audiences for influencer campaigns, they tend to be smaller than the audiences estimated using follower counts, which leads to reluctance by some agencies and marketers who want to show bigger numbers, Gilbreath says.

Gilbreath has pushed for marketers to apply the same rigor to influencer campaigns that they're trying to apply to other digital advertising. He charges only for the paid distribution, which is media using the influencer-created content but paid for like other ads, not the harder-to-quantify "earned" views that are often based on follower counts. Still, he acknowledges that marketers and agencies alike are lured by the bigger, unverified "earned" numbers.

Tracking engagement rates with influencer posts is one way to guard against inflated follower counts, Yuki says, even if that doesn't provide an ironclad reach number. Shareablee vets its engagement rates by tracking the activity from individual user accounts, so if someone likes something, say, 600 times a day, their numbers don't count. This weeds out bots.

Another factor driving influencer growth: Amazon. Jessica Thorpe, president of Gen.video, says brands are looking for ways to economically produce video to go with e-commerce. Amazon, for example, noting the growing importance of video on rival Google's YouTube in e-commerce purchase decisions, has turned to Gen.video influencers to create videos for its own site to fuel Father's Day and holiday sales, Thorpe says.

But as influencer marketing becomes more refined, it could spell bad news for all those future Jenna Marbles. Matt Britton, CEO of Crowdtap, predicts that brands will soon stray away from using an army of micro-influencers and lean more into one big-name influencer or celebrity. The reason, he believes, is it helps with brand safety issues. Rather than having to put dozens of influencers through background checks, a brand can spend money on one person with a large following, he says. So Gen Z, keep it clean.

Contributing: Megan Graham

CORRECTION: An earlier version of this article said Bingdom Studios is a joint venture with MediaLink. It's not.