Social network Facebook became inaccessible across much of the globe for a time on Tuesday, before returning to normal service.
Millions of users were unable to access their accounts.
Users in some countries also had difficulties accessing photo-sharing app Instagram.
Facebook said it was "aware that many people are currently having trouble accessing Facebook and Instagram".
Facebook, Instagram and Tinder down. If Twitter goes down too people will have to actually start talking to each other...... #nooooooooo— lehmo (@lehmo23) January 27, 2015
An Instagram spokesman posted on Twitter: "We're aware of an outage affecting Instagram and are working on a fix. Thank you for your patience."
The sites appear to have been unavailable for about 40 minutes before coming back online.
Dating app Tinder and chat services Aim and Hipchat were also reported to have been affected by problems over the period.
A hacker group called Lizard Squad claimed responsibility for the outage. The group was linked to an attack on Malaysia Airlines' website on Monday and previously said it had caused Sony's PlayStation Network and Microsoft's Xbox Live gaming services to go offline.
However, the BBC understands that Facebook is investigating another potential cause.