“Fast growth means you are always behind,” says Facebook COO Sheryl Sandberg.

It’s a nice problem to have—and one that Sandberg faced in 2008 when she quit a powerful job at Google GOOG —vice president of online sales and operations—to join Facebook FB as the No. 2 to then-23-year-old CEO and founder Mark Zuckerberg.

Facebook had just 550 employees at the time. And it was Sandberg, who had built her business unit at Google from four to 4,000 people in five years, who knew how to scale a business…and has helped Zuckerberg do that, all the way to 11,000 employees at Facebook today. “You can never hire the right people too early,” Sandberg says about building for fast growth. “And you often hire them too late.”

While she has avoided interviews since May, when her husband, Dave Goldberg, died suddenly, Sandberg agreed to answer my questions by email for a story called “The Google Effect” in the current Most Powerful Women issue of Fortune. “The Google Effect” features prominent women currently at Google—such as YouTube CEO Susan Wojcicki (No. 19 in Fortune‘s 2015 MPW rankings) and new chief financial officer Ruth Porat (No. 26)—and notable ex-Googlers who, like Sandberg, quit the most desirable workplace in the universe (Google has been No. 1 on Fortune’s 100 Best Companies to Work For list for six years running) to join younger startups or to start their own companies.

And now these women are applying their Google-bred skills to help build such hot startups as Slack, Stripe and Uber. Stacy Brown-Philpot, who left Google in 2012 to be COO at TaskRabbit, says that Sandberg used to press upon her the importance of lining up successors for herself way in advance of promotions. “How can you go on without having someone in place who can do your job?” Sandberg asked Brown-Philpot when they worked together at Google. Remembering this, Sandberg explains: “You should always have several people who could succeed you—and then several people who can succeed them—so that you’re prepared for future challenges.”

“No team has ever had too many strong performers,” the Facebook boss adds. “Building your bench is one of the most important things a leader can do.”

For more on Fortune’s 2015 Most Powerful Women list, check out this video: