Sheryl Sandberg says her daughter once asked about the U.S. presidents, "Mommy, how come they're all boys?"

In a Facebook post following Hillary Clinton's acceptance of the Democratic nomination, Sandberg said she is hopeful no children will ever have to ask that question again.

"We are close to making my daughter's question a relic of the past," Sandberg wrote. "Watching this historic moment with my children, I am thinking about our past and their future."

Sandberg had worked in the Treasury Department during President Clinton's administration. She is also one of the country's most outspoken feminists. She founded the "Lean In" movement, titled after her book, in which she encourages women to support one another in the workplace and advocate for themselves to get promotions and pay raises.

It's not exactly a surprise that Sandberg announced her support for Hillary. (She had signaled her support in June.) She said Clinton would help American women achieve equal pay, equality in the workplace, and to build "a society where everyone has the chance to reach his or her potential, regardless of background, gender, race, or sexual orientation."

Sandberg also said Clinton's policies would acknowledge the changing American family dynamic, in which the number of single mothers have doubled since the early 1970s. Sandberg noted that 40% of families with single mothers live in poverty, and those numbers are higher for black and Hispanic families.

Yet she said her support for Clinton is multifaceted.

"My support for Hillary is not because she is a woman," Sandberg said. "It is because she is the most qualified candidate -- and she is the leader we need."

She said Clinton is the best choice on immigration, embracing diversity and the economy.

"Tonight, I am hopeful thinking about what it means for my children to watch Hillary Clinton accept the Democratic nomination for president of the United States and for me to able to tell them ‪#‎ImWithHer‬," Sandberg wrote."