Millions of new users joined the Edward Snowden-endorsed Signal on Monday, it said on Twitter. Telegram, whose functionality closely mirrors that of WhatsApp, surged 55 places to top the U.S. iPhone download chart, according to Sensor Tower.

Facebook suffered a six-hour outage that spanned its WhatsApp messenger, the main social network and photo-sharing app Instagram, shutting out many of its 2.7 billion global users. The situation idled some of the company’s employees and prompted a public apology from Chief Executive Officer Mark Zuckerberg.

Facebook’s loss turned into a gain for other social media. Twitter Inc.’s network stayed online, with Chief Executive Officer Jack Dorsey tweeting his endorsement of Signal as a suitable WhatsApp alternative and amplifying Snowden’s urging of his followers to move away from the Facebook-owned app. Thousands of rooms on audio-chat app Clubhouse had people talking about the outage, a spokeswoman for the service said, adding that all its main metrics rose.

Signal’s focus on privacy — expressed in its policy to not collect any data on its users and thus avoid situations where governments may pressure it to disclose personally identifiable information — has made it a favorite for secure communications. Telegram has also historically added millions of users in the wake of major WhatsApp outages.

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