Facebook's hours-long outage on Monday may have hurt the company, its founder, shareholders and many businesses that rely on the social juggernaut's services. But for its instant messaging rivals, it was a very good day.

Telegram founder and chief executive Pavel Durov said on Tuesday that his instant messaging app added a mind-boggling 70 million users yesterday in what he described as a "record increase in user registration and activity" for the service.

"I am proud of how our team handled the unprecedented growth because Telegram continued to work flawlessly for the vast majority of our users," wrote Durov on his Telegram channel. But the day wasn't so flawless.

"That said, some users in the Americas may have experienced slower speed than usual as millions of users from these continents rushed to sign up for Telegram at the same time," he added.

Telegram, which recently topped 1 billion downloads, had 500 million monthly active users as of early this year.

Signal, which competes with both Telegram and WhatsApp, also added new users. It said yesterday in a tweet that “millions of new users” had joined the app.

This isn't the first time Telegram and Signal have gained at the expense of their chief rival. The two added millions of users earlier this year as well when WhatsApp was struggling to explain exactly what its new privacy policy entailed.

"The smallest of events helped trigger the largest of outcomes," said Brian Acton, the executive chairman of Signal's holding company, of WhatsApp's debacle earlier this year, in an interview with TechCrunch.