Recommended: “Facebook, Instagram, WhatsApp go down,” by Clare Duffy, for CNN Business — filed under “Unhackable.”

“More than four hours after the outage started,” Ms Duffy wrote, “Facebook CTO Mark Schroepfer tweeted: ‘We are experiencing networking issues and teams are working as fast as possible to debug and restore as fast as possible.’

… ‘I don’t know If I’ve seen an outage like this before from a major internet firm,’ said Doug Madory, director of internet analysis at network monitoring firm Kentik.

For a lot of people, Madory told CNN, ‘Facebook is the internet to them.’

… the fact that a company of Facebook’s size and resources has been offline for over three hours suggests there is no quick fix for the issue ….

Shares of Facebook were down more than 5% in midday trading Monday, putting it on pace for its worst trading day in nearly a year.