"The business would not exist at its current size without social media,” said Conor Begley, co-founder of brand-marketing company Tribe Dynamics.

Instagram co-founder Kevin Systrom, Snapchat co-founder Evan Spiegel, Facebook co-founder Mark Zuckerberg and Twitter co-founder Jack Dorsey are all billionaires younger than 45.

Each of Jenner’s Instagram posts is worth about $1 million, according to Hopper HQ, a social media scheduler. Jenner’s posts include family photos and mirror selfies, in addition to new product announcements or promotions for blush, lip gloss and eye shadow. Jenner has 128 million followers on Instagram, and plenty of others on Twitter, Facebook and Snapchat.

“She’s a unique case in that she has a massive, massive audience,” Begley said. “That can be replicated for sure."