NEW YORK (AP) — The six-hour outage at Facebook, Instagram and Whatsapp was a headache for many casual users but far more serious for the millions of people worldwide who rely on the social media sites to run their businesses or communicate with relatives, fellow parents, teachers or neighbors.

When all three services went dark Monday, it was a stark reminder of the power and reach of Facebook, which owns the photo-sharing and messaging apps.

Around the world, the breakdown at WhatsApp left many at a loss. In Brazil, the messaging service is by far the most widely used app in the country, inst...