In just under a year, Twitter will celebrate its 10th birthday. The microblogging service is currently the 8th-most visited website on the internet, with over 280 million active users. By offering a free, open platform that allows instantaneous communication to huge audiences, Twitter is arguably the most efficient means of spreading information in the world.

Twitter’s great advantage is its openness. Anyone can join a hashtag at any moment, or send a message to another user on the platform. The only thing that is required for their message to be seen is for others to be watching. Unless there is a breach of the terms of service, there is no top-down editorial control over messages, and no third party stands between a message and its intended recipient(s). Little wonder that activists in authoritarian countries are so enamoured with the platform.

Unfortunately, unrestricted free speech will always have its malcontents. Authoritarian activists using Twitter’s API have found a way to install themselves as de facto moderators for thousands of users. Twitter’s reputation for openness and free discourse may now be under threat.

The past two years have seen the rise of ‘autoblockers’ – shared lists of pre-blocked Twitter users that others can sign up to. The ostensible purposes of these lists is to block ‘abusers and trolls’ so that subscribers do not need to encounter them. However, as we shall see, this is typically a mask for politically-motivated blacklisting.

Two of these tools, the Atheism Plus Block Bot and the GG Autoblocker have attracted over ten thousand subscribers each. Both claim to be simple anti-harassment devices. But a look beneath the surface reveals something much more McCarthyite.

The Block Bot, which we have previously reported on, rose to prominence during the online trolling panic of 2013. It claims to be a one-stop shop for blocking trolls and abusers. In practice, the people added to its lists tended to be activists, academics, bloggers, and ordinary Twitter users who fell on the wrong side of political schisms within Atheism. Richard Dawkins, for example, was added to the list as a ‘rapeapologist’ and a ‘transphobe’, despite being neither of those things. Some have accused the Block Bot of engaging in defamation.

The GG Autoblocker is arguably even worse than the blockbot. Whereas the Block Bot decides who to block based on individual reports, GG Autoblocker uses guilt by association. The autoblocker maintains a list of several blacklisted users, including Breitbart London associate editor Milo Yiannopoulos, and at one point, the feminist academic Christina Hoff Sommers. If other Twitter users follow too many of these individuals, they will be automatically added to the autoblocker. You don’t have to do anything or even say anything to become a target. If you follow the wrong people, you’ll be blocked.

Twitter users targeted by the two blocklists have had enough, and are taking to the #AreYouBlocked hashtag in large numbers to demand that the company takes action. Urged on by academic Christina Hoff Sommers and game developer Mark Kern, both popular targets for autoblockers, users have caused the #AreYouBlocked hashtag to trend globally. Using the League for Gamers’ blockchecker as well as the Block Bot’s own search function, they have also uncovered an astonishing range of accounts targeted by the blocklists.

The following are just a small selection of targeted users:

Richard Dawkins (@RichardDawkins) – Evolutionary biologist

Pope Francis (@pontifex) – Head of the Catholic Church

Christina Hoff Sommers (@CHSommers) – Feminist philosopher and academic

Louise Mensch (@LouiseMensch) – former Member of Parliament

Jon ‘JonTron’ Jafarai (@JonTronShow) – YouTube personality

Mark Kern (@Grummz) – CEO, MEK Entertainment

Cathy Young (@CathyYoung63) – Journalist

Brian Cox (@ProfBrianCox) – Physicist, TV personality

Paul Staines aka Guido Fawkes (@GuidoFawkes) – Journalist

Alexander Macris (@archon) – Co-founder, The Escapist and Senior Vice President, Defy Media

Jennifer Dawe (@GMShivers) – Game developer

The Fine Young Capitalsts (@TFYCapitalists) – Feminist game designing collective

Airport (@CHOBITCOIN) – Transgender meme activist

Ashe Schow (@AsheSchow) – Journalist

Chris Mancil (@ChrisMancil) – Head of player communications, EA games

Julie Bindel (@bindelj) – Feminist campaigner

Kate Smurthwaite (@cruella1) – activist and comedian

Milo Yiannopolous (@Nero) – Journalist

As you can see, the list includes business leaders, journalists, academics, religious figures, media CEOs, scientists and activists. At one point, even Barack Obama was on the Block Bot.

There appears to be no consistent principle governing these lists. Most of their targets are not “trolls” or “harassers” by any stretch of the imagination. In some cases, users have been added to the blocklists after they have received harassment.

The truth is, it’s not “harassment” that landed these people on blocklists, but political disagreements. Christina Hoff Sommers, Cathy Young, and Louise Mensch were all added because they are critical of mainstream feminist arguments. Richard Dawkins was reported to the Block Bot for, among other things, criticising Islam. Ashe Schow was reported to the Block Bot for writing articles critical of feminist narratives in the Washington Examiner. Brian Cox, ludicrously, was reported for retweeting an article defending free speech on campus.

Others, particularly in the gaming industry, are on the GG autoblocker purely because of who they follow on Twitter.

Whether it’s political intolerance or guilt by association, it is clear that the primary purpose of these lists is not to block ‘trolls’ or ‘abusers’. They are, rather, the digital equivalent of campus safe spaces – walled gardens for people afraid of intellectual and political challenge, or for people so sure of their convictions that disagreement is taken as an insult.

But blocklists are more dangerous than campus safe spaces. While student authoritarians may, if they’re lucky, prevent a ‘triggering’ speaker from reaching a few hundred students, Twitter blocklists cut off tens of thousands. Furthermore, by attaching labels such as ‘abuser’. ‘bigot’, and in some cases ‘racist’, ‘transphobe’ or ‘rape apologist’ to the blocklists, they are responsible for the kind of mass smearing that would make Joe McCarthy’s eyes water.

Blockbots don’t just block. They attack reputations, ostracize, and cut off business leaders, activists and journalists from whole sections of the public. They may be a particularly gullible section of the public, but it is still an issue that Twitter needs to address.

If Twitter becomes a dominated by walled-off echo chambers, its usefulness as a means of public communication will be diminished. If a business wants to kick off an ad campaign, will they go to Twitter, where they might be blocked by any number of people without their knowledge? Or will they opt for a more predictable platform, like Facebook or YouTube? The same goes for journalists looking to break a viral story, or activists trying to reach new supporters.

Dick Costolo, the CEO of Twitter, is well aware that people often seek to pass off political disagreement as harassment. In an interview with the New York Times in February, he explained his bemusement when people came to him with examples of “fairly rational political discourse” that they argued was harassment.

It’s good that Costolo is aware of the problem. But the politically intolerant are still finding ways to exploit Twitter’s API to isolate their targets. If Twitter wants to remain an open platform rather than a collection of walled gardens, it’s a problem that they will have to address.