MacKenzie Bezos has pledged to give half her estimated $36.6 billion fortune to charity. Her ex-husband hasn't, though!

The newly independent billionaire became one of the richest people in the world after she divorced Amazon CEO Jeff Bezos earlier this year. Now, she's signed onto an initiative called The Giving Pledge, which "invites" the world's wealthiest people to pledge half their money to philanthropic causes, either while they're alive or after their deaths.

Nineteen other people joined the initiative alongside Bezos, including WhatsApp founder Brian Acton and Coinbase CEO Brian Armstrong, according to an announcement from the organization. There are now 204 people committed to the pledge from a total of 23 countries. Notably absent, however, is Jeff Bezos, who has long been criticized for not signing on despite being the richest man in the world. (His net worth is roughly $114 million.)

"In addition to whatever assets life has nurtured in me, I have a disproportionate amount of money to share," MacKenzie wrote in a letter posted to the Giving Pledge website. "My approach to philanthropy will continue to be thoughtful. It will take time and effort and care. But I won’t wait. And I will keep at it until the safe is empty."

The particulars of her pledge, including where the money will go and when, have yet to be announced.

After meeting and marrying Jeff in New York in the '90s, Mackenzie Bezos became an early Amazon employee. She retained 25 percent of their stock after the divorce, making her the world's third richest woman.

Last year, Mackenzie and Jeff pledged $2 billion of their $150 billion fortune to a charity supporting homelessness and education causes. That was just over 1 percent of the couple's wealth, and was conspicuously, um, smaller than the massive Giving Pledges of contemporaries like Bill Gates and Warren Buffett.

Additional reporting by Rachel Kraus.