Facebook, Instagram, WhatsApp and Facebook Messenger which went down on Monday afternoon with users worldwide reporting the crashes, have now been restored.
It was observed that the sites were restored around 10.45pm after about six hours of crash, cutting off billions of users around the world.
The online services all share an infrastructure and are owned by Facebook, which is owned by American billionaire Mark Zuckerberg.
It had been earlier reported that the crash started around 16:41BST, according to the website Downdetector. While just two problems were reported at 16:26, the site says, this had jumped to more than 27,000 just fifteen minutes later.