The platform's main subreddits for gaming, which has more than 37 million members; music, which has 32.3 million; and r/todayilearned, a page dedicated to sharing facts with 31.8 million users, are among those shutting down.

Pages dedicated to specific fandoms, including Harry Potter and Taylor Swift, have also decided to go offline.

While some communities taking part in the blackout have said they will return after 48 hours, others suggest they may not come back until Reddit backtracks on its upcoming changes.

What are the changes?

In April, Reddit announced it would start charging for developers to access its API - that stands for application programming interface.

It's what allows third parties to access information on the platform, most importantly so developers can run alternate smartphone apps for users who don't like Reddit's official one.

Until now, accessing the API was free for all - but charges will be introduced from 19 June.

Hold on, explain the API again…

Reddit's database is chock-full of everything that makes up Reddit - the posts, the comments, the profiles and so on.

Whenever you use a Reddit app, you are essentially asking the platform's API for permission to look at the posts, comments and profiles you want to see.

Like the staff at the entrance to a British museum, until now it had just waved you through with no cash required - but now it's demanding payment.

That's not an issue if you're going directly through Reddit, either via the web or its app, but it means for third-party developers the cost gets passed on to them.

And it's about to get expensive?

Reddit has not publicly revealed the exact pricing details, but the makers of the popular third-party app Apollo have claimed they would be charged more than $20m (£15.9m) a year at their current rate of API usage.

"The price they gave was $0.24 for 1,000 API calls," said a post on Apollo's own subreddit (a "call" being one of those aforementioned requests).

"With my current usage [that] would cost almost $2m per month, or over $20m per year."

Why can't people just use the official app?

What's crucial here is while Reddit launched way back in 2005, it didn't release its own app until 2016.

It meant that for years, users had to rely on third-party apps, and many became so used to their preferred choice that they've stuck with them and never turned to the official one.

Popular options include Apollo, Narwhal, Relay, and Infinity.

These apps differentiate themselves from the official Reddit one with their own aesthetic and features, and are shielded from unpopular changes Reddit makes to its own app.

Apollo, Reddit Is Fun, Sync, and ReddPlanet have all said they will be forced to shut down on 30 June, while others could follow suit or start charging their users to keep up with costs.

What have the subreddits going offline said?

Some communities that decided to go dark today did so after consulting with their members.

R/gaming said its members were "overwhelmingly in support of the blackout", as it said Reddit's API changes would make third-party apps "ludicrously more expensive for developers to run".

The music subreddit, which won't be accessible by members or general visitors for 48 hours, encouraged people to contact Reddit to make clear their opposition to the new policy.

Moderators of the Harry Potter subreddit have written an open letter, urging Reddit to reconsider the API charges to "preserve the rich ecosystem" that has developed around the platform.

The Taylor Swift subreddit, among others, has also raised concerns about the impact on users with disabilities, saying some third-party apps offer much better accessibility options than Reddit.

Reddit has said third-party apps with a focus on accessibility features, like screen reading for people who are blind, will be exempt from the API pricing changes, but they must be non-profit.

What has Reddit said?

Reddit has defended the impending API charges, saying the platform needs to be "fairly paid".

"Expansive access to data has impact and costs involved; we spend multi-millions of dollars on hosting fees and Reddit needs to be fairly paid to continue supporting high-usage third-party apps," said a statement to Sky News.

"Our pricing is based on usage levels that we measure to be comparable to our own costs."

The company said developers could make their maps "more efficient" to reduce the number of API calls required, adding that access would also remain free for moderator tools and bots.

It added: "We're committed to fostering a safe and responsible developer ecosystem around Reddit - developers and third-party apps can make Reddit better and do so in a sustainable and mutually-beneficial partnership, while also keeping our users and data safe."

Reddit chief Steve Huffman doubled down on the company's stance in a Q&A on the platform on Saturday, saying it needs to be "self-sustaining" and "can no longer subsidise commercial entities that require large-scale data use".

It comes as the company lays off 90 employees, about 5% of its workforce, to cut costs.