Earlier this week, WhatsApp co-founder Brian Acton urged students to delete their Facebook accounts, as reported by Buzzfeed News.

Acton made a rare public appearance alongside Ellora Israni, a former Facebook employee who founded She++. The two were speaking at Stanford University to students taking Computer Science 181. The undergraduate course focuses on” tech companies’ social impact” and their “ethical responsibilities.”

During his talk, Acton brought up the issue of moderation, which has become a big question in the tech world — especially for social media companies.

Tech companies continue to face struggles around moderating content. Although that’s something WhatsApp didn’t have to deal with because its encryption made it so no one could monitor what’s said on the app.

“I think it’s impossible,” Acton said in regards to moderation on other platforms. “To be brutally honest, the curated networks — the open networks — struggle to decide what’s hate speech and what’s not hate speech. … Apple struggles to decide what’s a good app and what’s a bad app. Google struggles with what’s a good website and what’s a bad website. These companies are not equipped to make these decisions.”

Acton went on to add, “And we give them the power. That’s the bad part. We buy their products. We sign up for these websites. Delete Facebook, right?”

Acton also criticized Facebook in other ways, referring to it as “a bit of a monoculture.” It’s apparent that he has been pretty vocal about his critiques  for a minute.

He originally left the company in 2017 — a decision that cost him $850 million — because Facebook wanted to monetize Whatsapp. In an interview, Acton told Forbes, “At the end of the day, I told my company. I told my users’ privacy to a larger benefit. I made a choice and a compromise. And I live with that every day.”

This isn’t the first time Acton has urged people to delete their Facebook accounts. When the Cambridge Analytica scandal originally broke last year, Acton tweeted, “It is time. #deletefacebook.”

He hasn’t tweeted another thing since.