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  • Age
    27
  • Title
    Founder and CEO
  • Company
    Tumblr

When Karp dropped out of high school at age 15, it wasn’t without a life plan: “I wanted to drive software, and I wanted to build stuff on the Internet,” he recalls. The rest is history: He built Tumblr, which hosts over 136 million blogs, and sold it to Yahoo earlier this year for $1.1 billion. Under its new parent — and CEO Marissa Mayer (see No. 1) — Tumblr is reportedly aiming for $100 million in sales this year from advertising, up from an estimated $13 million generated last year.

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