Marissa Mayer has been been playing defense for two years. As CEO of Yahoo, she has weathered critical attacks on her sleep habits, telecommuting policies, Vogue photo shoots, and most recently, her acquisitions. Yahoo  YHOO 1.69% has acquired approximately three dozen companies under Mayer’s watch, and activist investor Starboard Value posits the company has little to show for them, particularly the largest, her $1.1 billion deal for Tumblr.

This week, Mayer addressed those concerns on Yahoo’s unusually interesting earnings call. Unlike most of Yahoo’s consumer web products, Tumblr is culturally relevant with young people and growing, with 428 million users, a 40% increase over last year. But more importantly to shareholders, Tumblr, which had just $13 million in revenue in 2012 (it was acquired in 2013), is now on track to earn $100 million in ad revenue next year, Mayer said.

“I hope we blow past that,” Tumblr founder David Karp told Fortune in an interview.

More interesting than Tumblr’s surge in revenue and users, though, is what Mayer has planned for the social media site: She wants to make it into a YouTube competitor.

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