WhatsApp is hugely popular. Facebook purchased the service and its 450 million registered users in 2014 for $19 billion, and by February 2016, WhatsApp had ballooned to more than 1 billion monthly users. Just today, Facebook CEO Mark Zuckerberg announced WhatsApp Status, which operates a lot like Instagram Stories, has more than 175 million users. Compare that with Snapchat, the OG disappearing-message service: In February, the company reported a userbase of 161 million.
This isn't the only bad news in the tech industry today: Google had to shut down a massive and fairly sophisticated phishing scam that disseminated fake Google Docs links in a bid for users' account information.
Update: WhatsApp is back. At 8PM ET, a spokesperson provided Engadget the following statement: "Earlier today, WhatsApp users in all parts of the world were unable to access WhatsApp for a few hours. We have now fixed the issue and apologize for the inconvenience."