Signal said on Friday it was experiencing technical difficulties and working to restore the service, as it dealt with a flood of new users after rival messaging app WhatsApp announced a controversial change in privacy terms.
Along with another encrypted app, Telegram, Signal has been the main beneficiary of online outrage around the policy changes announced by WhatsApp last week. Telegram said on Wednesday it had surpassed 500 million active users globally.
"We have been adding new servers and extra capacity at a record pace every single day this week, but today exceeded even our most optimistic projections," said Aruna Harder, Chief Operating Officer of Signal Messenger.
Currently, more than 1,300 users are still facing issues, according to outage tracking website Downdetector.
The non-profit Signal Foundation based in Silicon Valley, which currently oversees the app, was launched in February 2018 with Brian Acton, who co-founded WhatsApp before selling it to Facebook, providing initial funding of $50 million.