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By Anita Balakrishnan, CNBC

Facebook has 4 million advertisers, up from 3 million in March, said Sheryl Sandberg, Facebook's chief operating officer, on Tuesday.

"I think that's because businesses know that consumers are spending time on mobile, and they need to be there," Sandberg said.

Sandberg's comments came as part of a CNBC interview amid New York City's Advertising Week, an event for marketing, brand, advertising and technology professionals. Sandberg said that video is "exploding" and especially important in Facebook's dominance of the mobile platform, although the company hasn't focused on monetization for products like Facebook Live.

Sandberg's information comes after a recent report revealed the social media company overestimated average viewing time for video ads on its platform for two years. While Sandberg said she can't promise Facebook will never make another mistake, she said that Facebook fixed the problem and explained it to clients.

"We take any mistake really seriously and trust is so important," Sandberg said. "We also believe very strongly in third-party measurement. We're not just grading our own homework."

In a separate WSJ report earlier this year, massive ad spender Procter & Gamble said it would move away from some of Facebook's targeted advertising due to questions about its effectiveness. Sandberg said the move was interesting from a legacy company like P&G, that has seen advertising shift from platforms like print to radio to television, noting that she thinks "anytime there's a shift in a platform you have to establish new metrics."

"What [advertisers] have always looked for is who sees their ads, how they are spending their ad budgets, and then, is that moving products off shelves," Sandberg said. "What we're working on is all of that measurement so that we can go all the way through from what someone sees to to their purchases."