http://www.newsweek.com/facebook-engineers-reveal-cause-monday-outage-1635516

The sudden Facebook outage on Monday left hundreds of millions of people around the world without their go-to source for news, current events, recipes and local gossip. The outage crippled not only Facebook and its app, but also Instagram and WhatsApp.

The outage happened around 1 p.m. ET in the United States, and it wasn't restored until five hours later. Facebook engineers at 10 p.m. ET—nine hours after the outage and four hours after restoration—explained the reason behind it.