After a nearly six-hour outage that prevented the company’s 3.5 billion users from accessing its social media and messaging services, Facebook, Instagram, Messenger, and WhatsApp reopened late Monday afternoon Eastern time.

Facebook apologised but did not immediately explain what caused the outage, which was the largest ever recorded by web monitoring firm Downdetector.

The outage was the social media giant’s second setback in as many days, following a whistleblower’s accusation on Sunday that the company had repeatedly prioritised profit over cracking down on hate speech and misinformation.

As users flocked to competing apps such as Twitter and TikTok, Facebook shares fell 4.9 percent on Monday, their biggest daily drop since last November, as part of a broader selloff in technology stocks. Following the resumption of service, shares gained about half a percent in after-hours trading.

“To every small and large business, family, and individual who depends on us, I`m sorry,” Facebook Chief Technology Officer Mike Schroepfer tweeted, adding that it “may take some time to get to 100%.”

Several Facebook employees who declined to be identified said they believed the outage was caused by an internal error in how internet traffic is routed to the company’s systems. Employees claimed that the failures of internal communication tools and other resources that rely on the same network to function exacerbated the error.

Security experts said an inadvertent mistake or sabotage by an insider were both plausible.

“Facebook basically locked its keys in its car,” tweeted Jonathan Zittrain, director of Harvard`s Berkman Klein Center for Internet & Society.

On Monday, Twitter reported higher-than-normal usage, which caused some issues with people accessing posts and direct messages.

In one of the day`s most popular tweets, video streaming company Netflix shared a meme from its new hit show “Squid Game” captioned “When Instagram & Facebook are down,” that showed a person labeled “Twitter” holding up a character on the verge of falling labeled “everyone.”

After the service returned that “lots of people searched today how to run google ads for clients,” one member wisecracked in a Facebook group for ad buyers.

According to estimates from ad measurement firm Standard Media Index, Facebook, the world’s second-largest seller of online ads after Google, was losing about $545,000 in US ad revenue per hour during the outage.

However, previous downtime at internet companies had little long-term impact on revenue growth.

Facebook’s services, which include consumer apps like Instagram, workplace tools it sells to businesses, and internal programmes, went dark at noon Eastern time (1600 GMT). Around 5:45 p.m. ET, access began to be restored.

Soon after the outage began, Facebook acknowledged that users were having difficulty accessing its apps, but did not provide any details about the nature of the problem or the number of users affected.

The error message on Facebook’s website indicated a problem with the Domain Name System (DNS), which allows web addresses to direct users to their destinations. In July, a similar outage at cloud company Akamai Technologies Inc knocked out multiple websites.

Frances Haugen, a product manager on Facebook’s civic misinformation team, revealed on Sunday that she was the whistleblower who provided documents supporting a recent Wall Street Journal investigation and a US Senate hearing last week on Instagram’s harm to teen girls.

Haugen was due to urge the same Senate subcommittee on Tuesday to regulate the company, which she plans to liken to tobacco companies that for decades denied that smoking damaged health, according to prepared testimony seen by Reuters.