Facebook, Instagram and WhatsApp went offline for users across the globe, the social media giant said on Monday.
Reuters could not immediately confirm the issue affecting the services, but the error message on Facebook's homepage suggested a problem with the Domain Name System, or DNS.
Security experts tracking the situation said the outage could have been triggered by a configuration error, which could be the result of an internal mistake, though sabotage by an insider would be theoretically possible.
An outside hack was viewed as less likely.
Facebook acknowledged users were having trouble accessing its apps but did not provide any specifics about the nature of the problem or how many were affected by the outage.
The company said "We're working to get things back to normal as quickly as possible and we apologize for any inconvenience."
Shares of Facebook, which has nearly 2 billion daily active users, fell more than five percent in afternoon trading on Monday.