Website monitoring group Downdetector said Monday’s outage was the largest such failure it had ever seen. The outage also comes as Facebook contends with damaging revelations from an ex-employee.

Mark Zuckerberg commented on his company’s global outage, in a Facebook post on his restored site. Zuckerberg offered no explanation for what had happened, merely saying: “Facebook, Instagram, WhatsApp and Messenger are coming back online now.

Sorry for the disruption today — I know how much you rely on our services to stay connected with the people you care about,” he posted.