Some Facebook, Instagram and WhatsApp users cannot upload photos, videos and files.
Facebook, which owns all three apps, said it was aware of the issue and was “working to get things back to normal as quickly as possible”.
The company’s main social network, its two messaging apps and image-sharing site Instagram have billions of users.
Rival platform Twitter also had issues, with some users not able to send direct messages or receive notifications.
The company apologised for the inconvenience, tweeting: “We’re currently having some issues with DM delivery and notifications.
“We’re working on a fix and will follow up as soon as we have an update for you.”
Users were still able to post on Twitter and #instagramdown began trending as many people used the site to highlight the problems with its competitors.
The Facebook Messenger app, which is often installed separately, is also affected.
Facebook has more than 2.3 billion monthly active users and Instagram has one billion.