Those in the UK, US and Europe appear to be hardest hit in the latest in a string of outages.
Reports have come in from further afield, including Australia, Brazil, Colombia, Japan, Mexico, The Philippines, Russia, Singapore, South Africa and Uruguay.
Many took to rival platform Twitter to highlight the problem, which seems to be preventing images and videos from loading properly.
Facebook said it was working to resolve the error as the hashtag #instagramdown began to climb up the trending list on Twitter.
On WhatsApp, some users noticed image and video files were not able to send, while others on Instagram claimed they could not see images on their feed.
“We’re aware that some people are currently having trouble uploading images, videos and other files on our apps,” a spokeswoman for Facebook said.
“We’re working to get things back to normal as quickly as possible.”
Facebook has more than 2.3 billion monthly active users and Instagram has one billion.