FACEBOOK, Instagram and WhatsApp at least partially reconnected to the global internet late on Monday afternoon, nearly six hours into an outage that paralyzed the social media platform.

Facebook and its WhatsApp and Instagram apps went dark at around noon Eastern time (1600 GMT), in what website monitoring group Downdetector said was the largest such failure it had ever seen.

Around 5:45 pm ET, some Facebook users began to regain partial access to the three apps.

The outage was the second blow to the social media giant in as many days after a whistleblower on Sunday accused the company of repeatedly prioritizing profit over clamping down on hate speech and misinformation.

"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%."

Shares of Facebook, which has nearly 2 billion daily active users, fell 4.9% on Monday, their biggest daily drop since last November, amid a broader selloff in technology stocks.

Security experts said the disruption could be the result of an internal mistake, though sabotage by an insider would be theoretically possible.

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

Soon after the outage started, Facebook acknowledged users were having trouble accessing its apps but did not provide any specifics about the nature of the problem or say how many users were affected by the outage.

Several Facebook employees who declined to be named said that they believed that the outage was caused by an internal routing mistake to an internet domain that was compounded by the failures of internal communication tools and other resources that depend on that same domain in order to work.

Facebook, which is the second largest digital advertising platform in the world, was losing about $545,000 in U.S. ad revenue per hour during the outage, according to estimates from ad measurement firm Standard Media Index.