Facebook has admitted that its users are having “trouble accessing our apps” after its network of services, including Instagram and WhatsApp, were hit by an outage in several countries, including the UK and the US, affecting millions of people. Facebook, Instagram and WhatsApp became inaccessible for large numbers of people at about 5pm UK time, with Downdetector.com citing reports of problems from millions of social media users around the world. A map on the site showed, for instance, reports of outages from cities across the US, UK and Australia. By Monday evening it appeared that some of Facebook’s services were beginning to come back online, but a spokesperson reportedly said it would be a while before full restoration. In a blogpost, Downdetector said the outage was global and the largest it had ever seen. “Facebook is currently experiencing a rarely seen global outage that is taking out Facebook, Instagram, WhatsApp and Facebook Messenger.” As of 1pm PDT, Downdetector said it had seen more than 10.6m problem reports from around the world, with the majority coming from the US, Germany, the Netherlands and the UK. “We are also seeing an increase in reports across many other online sites and services as the Facebook outage causes cascading impacts,” the blogpost said. Shares in Facebook, which has nearly 2 billion daily active users, opened lower after Sunday’s TV interview with whistleblower Frances Haugen and slipped further to trade down 4.9% in afternoon trading on Monday. They were on track for their worst day in nearly a year, amid a broader selloff in technology stocks on Monday. Mark Zuckerberg’s net worth fell by more than $6bn on Monday, Bloomberg reported. The Facebook CEO’s worth has dropped from $140bn to $121.6bn in recent weeks. Haugen, a former Facebook employee, accused the company of putting profit over safety, after coming forward as the person who leaked a cache of internal documents that have placed the tech firm in its worst crisis since the Cambridge Analytica scandal. On Twitter, the Facebook spokesperson Andy Stone said: “We’re aware that some people are having trouble accessing our apps and products. We’re working to get things back to normal as quickly as possible, and we apologise for any inconvenience.” The Instagram site was displaying a 5xx server error message, while the Facebook site was telling users: “Sorry, something went wrong.” WhatsApp said on Twitter: “We’re aware that some people are experiencing issues with WhatsApp at the moment. We’re working to get things back to normal and will send an update here as soon as possible. Thanks for your patience.” Twitter itself had a little fun at its rivals’ expense. “Hello literally everyone,” said its account. Facebook engineers are responding to a company data center in California to fix the outage, which has also disrupted internal systems employees use for work, the Verge reported. Adam Leon Smith, of BCS, the Chartered Institute for IT and a software testing expert, said: “The outage is caused by changes made to the Facebook network infrastructure. Many of the recent high-profile outages have been caused by similar network-level events. “It is reported by unidentified Facebook sources on Reddit that the network changes have also prevented engineers from remotely connecting to resolve the issues, delaying resolution. “Notably, many organisations now define their physical infrastructure as code, but most do not apply the same level of testing rigour when they change that code, as they would when changing their core business logic.” According to reports, part of the problem was with the DNS, or domain name system, which turns website names such as theguardian.com into numeric addresses that can be understood by machines. These allow the users’ computer to connect to the destination web server and the website users are looking for. The journalist Sheera Frenkel, the author of a recently published book about Facebook, said the network outage was so severe that Facebook staff had been unable to enter their own company buildings because their passes were not working. Monday’s outage is the first serious incident for Facebook since 2019, when it suffered a series of failures, including on 13 March, when some users around the world could not access Facebook, Instagram and WhatsApp for more than 24 hours.