Signal is a private messaging app that has commitments not to share data and allows users to keep their messages hidden.

It has been newly boosted after an update to WhatsApp’s terms that allows the app to share data with parent company Facebook. WhatsApp users must agree to the new rules or be kicked off the app entirely.

The problems came as people including Elon Musk suggested users should switch to Signal instead. In a post that was shared 28,000 times, Mr Musk simply wrote: “Use Signal”.

As users looked to switch, Signal had trouble keeping up with the influx of new users and sending them the verification codes required to sign up. Signal links accounts with phone numbers and send users a text message to verify they own that number.

“Verification codes are currently delayed across several providers because so many new people are trying to join Signal right now (we can barely register our excitement),” Signal wrote on its official Twitter account.

“We are working with carriers to resolve this as quickly as possible. Hang in there.”

Mr Musk did not specifically point to the new WhatsApp terms or give any further information about why he was instructing people to use the new app. Over the course of the day he had shared jokes both criticising Facebook over the WhatsApp change, but also over its perceived role in the riots at the Capitol building.

The controversial part of WhatsApp’s new terms is the removal of an opt-out for data sharing with Facebook, and a new passage that explains how any shared data will be used.

“As part of the Facebook family of companies, WhatsApp receives information from, and shares information with, this family of companies,” the new privacy policy says.

“We may use the information we receive from them, and they may use the information we share with them, to help operate, provide, improve, understand, customize, support, and market our Services and their offerings.”

But WhatsApp has argued that the change to the terms is not part of an alteration to the way information is passed between the two companies.

“There are no changes to WhatsApp’s data-sharing practices in the Europe arising from this update,” Niamh Sweeney, WhatsApp’s director of policy in Europe, the Middle East and Africa, wrote in a series of tweets. “It remains the case that WhatsApp does not share European Region WhatsApp user data with Facebook for the purpose of Facebook using this data to improve its products or ads.”