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Facebook hits 4 million advertisers, COO Sheryl Sandberg says

Facebook has 4 million advertisers, up from 3 million in March, COO Sheryl Sandberg told CNBC's on Tuesday.

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

Sandberg spoke on the sidelines of Advertising Week, a New York event for marketing, brand, advertising and technology professionals. She 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.

Sheryl Sandberg
David A. Grogan | CNBC
Sheryl Sandberg

It comes after a Wall Street Journal report that 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 Journal 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. She said "anytime there's a shift in a platform you have to establish new metrics."

"When you think about what advertisers have looked for and measured, what they've always looked for is who sees their ads, how are they spending their ad budgets, and then, is that moving products of shelves, is that driving cars off lots?" she said. "And so, what we're working on, is all of that measurement, so that can go all the way through from what someone sees to their purchases."