The Signal app was downloaded almost 1.3 million times on Monday, according to data from Apptopia, a tracking firm. The encrypted-messaging app had been downloaded an average of 50,000 times a day prior to Musk's tweet. The New York Times earlier reported the Apptopia data.
Read more: Everything to know about Signal
A Signal spokesperson said the report undercounted the number of downloads the service is experiencing.
"Right now I can say that all those app-analytics firms severely under report numbers from Signal because we don't have any trackers or analytics like other apps," the spokesman said. He said the company doesn't share its numbers publicly.
Signal also attributed a temporary Friday outage to the user surge.
"While we have been working hard all week to keep up with all the new people switching over to Signal, today exceeded even our most optimistic projections. We are working hard to resolve (the issue)," a spokesman told CNET in an email.
The surge in downloads comes as skepticism over Big Tech rises in the US and other countries. Many users are concerned Big Tech platforms censor their voices, a worry that was exacerbated after Facebook, Twitter and YouTube blocked President Donald Trump's use of their platforms after last week's deadly riot at the US Capitol building. Many users are skeptical of being tracked, after Facebook-owned WhatsApp changed its policies on Jan. 4.
Signal is the most downloaded iOS app in 70 countries and the most downloaded Android app in more than 50 countries, the spokesman said, citing India, West Africa and Brazil as top markets.
Neither Apple nor Google immediately responded to requests for comment. On Thursday, Signal topped App Annie's list of free iOS downloads as well as the Play Store's chart of free apps.
WhatsApp recently changed its policies to give it permission to see communications between users and businesses within the app. WhatsApp can use the information for marketing or Facebook advertising. The changes don't affect personal communications between users of the app.
Earlier this week, WhatsApp published an FAQ aimed at clarifying its data collection policy, emphasizing that neither it nor Facebook can see users' private messages or hear their calls. Following mounting privacy concerns, WhatsApp announced Friday that it would delay the rollout of its new policy by three months.
"We're now moving back the date on which people will be asked to review and accept the terms," the company said in a blog post. "No one will have their account suspended or deleted on February 8. We're also going to do a lot more to clear up the misinformation around how privacy and security works on WhatsApp. We'll then go to people gradually to review the policy at their own pace before new business options are available on May 15."
Signal's lead in India comes as California-based WhatsApp faces its first legal challenge in Indian courts over privacy concerns. With 400 million users, India is WhatsApp's largest market, according to a Thursday report from Reuters.
Telegram's download numbers are likewise booming. In December, the company was nearing 500 million users, a mark it passed on Tuesday, according to a statement from co-founder Pavel Durov.
"We've had surges of downloads before, throughout our 7-year history of protecting user privacy," Durov said. "But this time is different."
Durov, the company's primary financial backer, had earlier said Telegram "needs at least a few hundred million dollars per year to keep going" and plans to launch its own Ad Platform to generate revenue. It'll also sell new premium features to users in 2021.The company had previously raised $1.7 billion to develop blockchain tokens, though that attempt fizzled. Telegram offered to return $1.3 billion of the funds back to investors.
Signal is funded by the nonprofit Signal Foundation. The service has no ads and is offered free of charge. Brian Acton, a co-founder of WhatsApp who left following its acquisition by Facebook, initially bankrolled the foundation with $50 million.
After the shutdown of Parler, a heavily right-wing social network, some extremists have begun searching for new communications platforms. Telegram confirmed to CNN's Brian Fung on Wednesday that it had already blocked dozens of accounts associated with violent extremism.
Telegram didn't immediately respond to a request for comment.