Abdulaziz al-Dukhail, who had served as deputy finance minister, went missing in April along with at least two other public intellectuals also believed to be in detention for their implied criticism of the state.

In the following months, separate claims surfaced that a Twitter data breach by Saudi infiltrators in 2015 resulted in a wave of "enforced disappearances" of regime critics, many with anonymous accounts on the social media platform.

The cases illustrate how Saudi Arabia, which accounts for the most Twitter users in the Arab world, has sought to harness the power of the platform to promote its ambitious reforms while also aggressively seeking to tame free expression.

The three public figures dropped from view after expressing sympathy over the death of jailed activist Abdullah al-Hamid, according to family members and two campaign groups including the London-based ALQST.

Hamid, a veteran activist, died after suffering a stroke in detention while serving an 11-year sentence, sparking a torrent of criticism from international campaigners.

Dukhail's exact whereabouts are not known and authorities have not revealed any formal charges, his son Abdulhakim al-Dukhail told AFP.

"Why was he taken? What was his crime?" said Abdulhakim, currently based in Paris.

"Is he in jail just for a tweet?".

Saudi authorities did not respond to AFP's request for comment.

Crackdown

The detentions mirror an offline clampdown on dissent, with activists, bloggers and even royal family members arrested in recent years as Crown Prince Mohammed bin Salman bolsters his grip on power.

Saudi Arabia has stepped up arrests under a loosely worded anti cyber-crime law, which campaigners including Amnesty International say criminalises online criticism of the government.

"A simple tweet can land you in jail in Saudi Arabia with no access to a lawyer for months, maybe years," said Lynn Maalouf, Amnesty's Middle East research director.

Further raising concern is a 2015 Twitter data breach by Saudi moles, which led to the unmasking and arrests of anonymous critics of the government on the platform, according to family members and two lawsuits against the company.

The US justice department has charged two former employees with spying for the Saudi government as they accessed data on more than 6,000 accounts while looking for users "critical of the regime".

"Such private user information included their email addresses, phone numbers, IP addresses, and dates of birth," the justice department said last year, warning the data may have been used to locate the users.

One of those unmasked was Abdulrahman al-Sadhan, a 36-year-old employee of the humanitarian group Red Crescent who voiced opinions over human rights and social justice issues on an anonymous Twitter account, according to his family.

He was picked up from his Riyadh office by Saudi Arabia's secret police in March 2018, said his San Francisco-based sister Areej al-Sadhan.

Two years after he disappeared, he was allowed a brief call to a relative and revealed he was being held at the high-security Al-Ha'ir prison near Riyadh.

"It was his first and only call -- it lasted less than a minute," Areej told AFP.

"Someone behind him said 'your minute is up'. There was no goodbye, no 'talk to you later', no closure. The line was cut."

Social media weaponised

Two Saudi dissidents based in North America claimed in separate lawsuits against Twitter that their accounts were targeted in the breach, which endangered the lives of their associates in the kingdom.

One of them, Ali al-Ahmed who heads the Washington-based think tank Institute for Gulf Affairs, filed an amended complaint in August lashing out at Twitter over its "abject failure" to protect his account.

Ahmed's lawyer provided AFP with a list of eight Saudis who were in contact with him through anonymous Twitter accounts, claiming they ended up jailed, missing or dead after the breach.

Twitter did not respond to a request for comment.

In recent years, the online giant has deleted thousands of "state-backed" Saudi accounts, citing a violation of the platform's manipulation policies.

Saudi Arabia, which market research firm Statista says has around 12 million Twitter users, has seen a growth in online armies of self-styled patriots who cheerlead government policy and attack critics.

They rose as part of a policy driven by former royal court advisor Saud al-Qahtani, who earned nicknames such as "lord of the flies" for managing an electronic army.

"Saudi's digital authoritarianism... is egregious in its audacity," said Marc Owen Jones, author of the upcoming book "Digital Authoritarianism in the Middle East".

"Over the past few years, Saudi-connected entities have successfully utilised and penetrated Twitter to the extent that Twitter itself has become a weapon of authoritarian rule."