Craig Newmark, the founder of Craigslist, is on a mission to shame the ultra-wealthy into doing what he’s already doing: giving away their fortunes to causes that actually matter.

In a recent New York Times op-ed, Newmark laid out his case for why billionaires should stop hoarding their cash and start donating it to nonprofits. He’s already put his money where his mouth is, donating over $450 million to charitable causes over the past decade. Last year, he signed the Giving Pledge, a commitment to donate up to $1 billion before he dies or through his will.

“When I started Craigslist in the mid-1990s I never thought I’d become rich. But I did”, Newmark wrote. “A lot of people in tech around that time also got lucky. Millions, even billions, were made simply by being in the right place at the right time”.

The Giving Pledge was founded by Warren Buffett, Bill Gates, and Melinda French Gates, and it’s basically an agreement among the ultra-wealthy to give away most of their fortunes to charitable causes. So far, around 250 people have signed on.

Newmark says he decided to go public about his giving because he wanted to convince other billionaires to follow suit. And honestly, good for him for that.

But here’s where it gets spicy: some billionaires have actually criticized the Giving Pledge, calling it foolish to give away your fortune, especially to what they call “leftwing” nonprofits. Newmark wasn’t having it. “It makes no sense to me that others with this kind of money would criticize anyone doing this”, he wrote.

The thing is, the Giving Pledge lets donors choose which causes to support. So if some billionaires want to fund conservative causes instead, they can. It’s not like Newmark is forcing anyone to donate to specific nonprofits.

Newmark’s own donations support causes like independent journalism, support for active military personnel and veterans, cybersecurity, and, we’re not joking, pigeon rescue groups. He describes himself as a programmer, not a businessman, and kept Craigslist intentionally simple and mostly free for users. He only introduced limited fees for commercial postings because he felt it was unfair for companies to profit from the platform without contributing anything back.

“When I became accidentally wealthy it meant a lot to me that I could help people who were less lucky”, Newmark explained.

As Craigslist celebrates its 31st year this year, Newmark’s push for billionaire philanthropy serves as a reminder that some wealthy people actually do care about making a difference. The real question now is: will his words inspire other billionaires to finally open their wallets, or will they continue to hoard their wealth while the rest of us struggle?