The users of highly-encrypted Signal app faced trouble sending messages and the company said on Saturday that it was still working to bring additional capacity online to handle peak traffic levels.
The problem started late Friday and the company first acknowledged it may be due to technical difficulties.
Signal allows for secure and encrypted video, voice and text communication, but users were unable to send any messages.
"We have been adding new servers and extra capacity at a record pace every single day this week nonstop, but today exceeded even our most optimistic projections. Millions upon millions of new users are sending a message that privacy matters. We appreciate your patience," it said in a series of tweets.
As WhatsApp started sending notifications to its users asking them to either accept its new policy or see their accounts removed from February 8, it came as a blessing in disguise for its rivals as Signal as it saw a massive flow of new users.
The rush led to extra burden on Signal servers.
"We are making progress towards getting the service back online. Privacy is our top priority, but adding capacity is a close second right now," it said.
"We are still working as quickly as possible to bring additional capacity online to handle peak traffic levels".
The encrypted messaging service has climbed to the top spot in the free apps category of the App Store in multiple countries, including India.