That small mistake had big consequences. Instagram users couldn't view other profiles, WhatsApp users couldn't send messages, and news feeds across Facebook's main app went blank. Downdetector, which likens itself to a weather report for the internet, said it had received 7.5 million problem reports about Facebook's apps. In comparison, widespread problems on YouTube in October prompted just 2.7 million reports. Downdetector measures service interruptions in part by counting reports from users who are experiencing problems. "Never before have we seen such a large-scale outage," said Tom Sanders, a co-founder of Downdetector. Early Thursday, Facebook was able to pull most of its systems back online. The company is still trying to figure out how that error reverberated throughout its network.