Kylie Jenner is the world's youngest billionaire

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Kylie Jenner has become the world's youngest self-made billionaire.

The 21-year-old reality star insists she "didn't expect" to be so successful when she launched her beauty business Kylie Cosmetics - which Forbes magazine values at at least $900 million - four years ago.

She told the financial publication: "I didn't expect anything. I did not foresee the future. But [the recognition] feels really good. That's a nice pat on the back."

Most of the brand's marketing is done through social media - where the brunette beauty announces product launches, previews new items and tells fans which shades she's wearing each day - to her 175 million-plus fan base across Instagram, Facebook and Twitter and Kylie believes it's been essential to her success.

She added: "It's the power of social media. I had such a strong reach before I was able to start anything."

Kylie has overtaken Facebook founder Mark Zuckerberg, who previously held the title of youngest self-made billionaire from the age of 23.

The make-up mogul signed an exclusive distribution deal in November last year with beauty store chain Ulta to stock her $29 lip kits - a matte liquid lipstick and matching lip liner - into the company's 1,000-plus stores across 50 states in the US, having previously sold only online or occasionally at pop-up shops.

Following her deal with the beauty company, Kylie Cosmetics sold $54.5 million worth of products in just six weeks, according to estimates from Oppenheimer.

Kylie owns her entire company, which she started in 2015, and employs just seven full-time and five part-time employees, while manufacturing and packaging is outsourced to Seed Beauty and her mum and manager Kris Jenner helps run the brand's finances and PR.

Kylie Cosmetics' revenue climbed 9% to an estimated $360 million last year.