When Mark Zuckerberg and Priscilla Chan pledged in 2015 to give away 99% of their Facebook shares, the stake was worth roughly $45 billion. Today, Zuckerberg's fortune exceeds $200 billion — and the couple's three daughters are still not slated to inherit most of it. Because the pledge covers a percentage of their holdings rather than a fixed dollar figure, the amount ultimately directed to charity grows with the stock itself.

That decision places the Meta chief executive and Chan, a pediatrician, in a cohort of ultra-wealthy households that includes Bill Gates, Warren Buffett, and Michael Bloomberg, whose children are not expected to inherit the bulk of the family balance sheet. The cohort overlaps heavily with the Giving Pledge, the 2010 campaign founded by Buffett and Gates, which Zuckerberg and Chan joined five years before launching CZI.

A pledge that began with a firstborn

The couple announced their decision in December 2015, after the birth of their first child, Maxima. In an open letter to their newborn daughter, they introduced the Chan Zuckerberg Initiative (CZI) and committed to giving away 99% of their Facebook — now Meta — shares during their lifetimes.

"Like all parents, we want you to grow up in a world better than ours today," they wrote. "Our society has an obligation to invest now to improve the lives of all those coming into this world, not just those already here."

The pledge stands out against a broader backdrop: an estimated $124 trillion is expected to pass to younger generations as baby boomers hand down their portfolios.

California's billionaire tax looks set to miss his fortune

Zuckerberg's wealth also appears unlikely to be touched by California's proposed "billionaire tax." Proposition 40, a one-time 5% levy on the net worth of the state's billionaires, was trailing 55% to 45% as of Nov. 6, with ballots still being counted.

Proposition 42, a billionaire-backed countermeasure that would nullify the tax and bar new wealth taxes in California, was leading with about 54% support. Once counties finish counting, the final tallies will determine which vision of California's tax code prevails.

Zuckerberg had told Meta's board he was weighing a move out of the state over the measure. The company responded by creating a new class of stock that he could sell to cover a potential bill.

Why CZI is an LLC, not a foundation

The structure of the couple's philanthropy is notable. CZI is organized as a limited liability company rather than a conventional private foundation, an arrangement that gives the pair flexibility over timing and cause areas and allows them to invest in for-profit companies and fund policy advocacy. Private foundations, by contrast, operate under federal rules that include minimum annual payouts and limits on lobbying and political activity — obligations an LLC structure does not carry.

Gates took the foundation route instead. The Gates Foundation now plans to close in 2045 after spending down roughly $200 billion.

Gates has nonetheless cited Zuckerberg as a model of giving, telling Fortune in a conversation last year: "There are exemplars like Mark Zuckerberg, you know, through the Chan Zuckerberg Initiative, is committed to give over 90% of his wealth away and he's off to a very good start."

"He's someone I talked to about a lot of things, including philanthropy, and they do education research; we partner on some of that," Gates added.

From housing to AI-driven biomedical research

CZI's early years centered on broad social issues, including housing affordability in California and criminal justice reform. More recently, the organization has shifted its core funding toward technology-driven priorities, particularly AI-enabled biomedical research.

In the letter, the couple tied the commitment directly to their daughter. "We will do our part to make this happen, not only because we love you, but also because we have a moral responsibility to all children in the next generation," they wrote.

What the pledge doesn't require

The pledge does not require Zuckerberg to live like a monk. He still holds a roughly 13% stake in Meta — enough, through the company's multi-class share structure, to give him majority voting control of the company. He has also assembled one of the most expansive real estate portfolios of any American billionaire, including a recent purchase in Miami.

Near the end of the letter, the couple addressed their daughter by name. "Max, we love you and feel a great responsibility to leave the world a better place for you and all children," they wrote.

Their final line: "We can't wait to see what you bring to this world."

This story was originally featured on Fortune.com.