Mark Zuckerberg’s “Twitter killer” Threads app has reached 100m sign-ups in less than five days.

The Twitter rival has grown rapidly since its launch last Wednesday, with Zuckerberg seeking to woo users with a promise that the platform would focus on “kindness”.

Twitter’s Elon Musk has responded with a threat to sue Meta, the parent company of Threads, over its launch of a “copycat” app.

Writing on his Threads account, Zuckerberg said the new app had reached 100 million users over the weekend. By contrast Instagram, owned by Zuckerberg’s Meta empire, took a week to reach 100,000 users when it launched as an independent platform in 2010.

“Threads reached 100 million sign-ups over the weekend,” wrote Zuckerberg. “That’s mostly organic demand and we haven’t even turned on many promotions yet. Can’t believe it’s only been five days!”

Threads users need an Instagram account to log into the new app, with the ability to transfer over the accounts they follow on Instagram to the new platform. Despite Zuckerberg’s focus on kindness, the US non-profit group Media Matters said rightwing figures have signed up to Threads including the white nationalist Richard Spencer and Nick Fuentes, a white supremacist and outspoken antisemite. However, Meta has since taken down Spencer’s account.

Twitter has gone through a period of prolonged upheaval since Musk’s $44bn (£38bn) takeover last year, with the billionaire making a series of changes, such as swingeing job cuts and imposing viewing limits.

Mike Proulx, the research director at the analysis firm Forrester, said the Threads launch so far had been a “case study on what to do right”. He added: “The meteoric rise of Threads, in just five days, demonstrates just how many people have longed for an alternative to what Twitter has devolved into.”

However, Proulx said Threads needed to prove that it can manage sustained growth and getting people to use it repeatedly. Meta, which also owns Facebook and WhatsApp, has delayed launching Threads in the EU amid doubts over how the bloc will regulate the app’s use of personal data.

On Sunday the web services firm Cloudflare claimed that website traffic to Twitter, which has more than 250 million daily users, according to Musk, was “tanking”. The Cloudflare managing director, Matthew Prince, posted a graph on Twitter showing a drop in Cloudflare’s domain server ranking for Twitter in mid-2023. The drop in early July appeared to coincide with Musk imposing viewing limits on Twitter and the launch of Threads.

Twitter has, however, retained the support of the senior Taliban leader Anas Haqqani, who endorsed the app over Meta’s Threads, which he said has an “intolerant policy”.

“Twitter has two important advantages over other social media platforms. The first privilege is the freedom of speech. The second privilege is the public nature & credibility of Twitter,” he said on Twitter.

Twitter has been contacted for comment.