A large number of subreddits, including some with tens of millions of followers, have switched over to private mode. The moderators of the subreddits, who are unpaid, are doing this in response to Reddit's plan to start charging third-party developers a fee to access its API.
Millions of Reddit users use apps like Narwhal and Apollo to access the site, because Reddit's own app is lousy. The creator of Apollo, Christian Selig, said the new API fees will total $20 million a year. Last week, he announced, "Apollo will close down on June 30th. Reddit's recent decisions and actions have unfortunately made it impossible for Apollo to continue. Thank you so, so much for all the support over the years."
The site Redditdark has an updated list of all the subreddits that have gone dark in protest so far. It includes r/funny with 40+ million followers, and r/aww, r/gaming, r/Music, r/Pics, r/science, and r/todayilearned, each with 20+ million followers.