Meta Platforms' Twitter rival Threads crossed 100 million sign-ups within five days of launch, CEO Mark Zuckerberg said on Monday, dethroning ChatGPT as the fastest-growing online platform to hit the milestone.

Threads has been setting records for user growth since its launch on Wednesday, with celebrities, politicians and other newsmakers joining the platform that is seen by analysts as the first serious threat to the Elon Musk-owned microblogging app.

"That's mostly organic demand, and we haven't even turned on many promotions yet," Zuckerberg said in a Threads post announcing the milestone.

The app's sprint to 100 million users was much speedier than that of OpenAI-owned ChatGPT, which became the fastest-growing consumer application in history in January about two months after its launch, according to a UBS study.

Read more: Meta launches Twitter rival app Threads

Twitter had nearly 240 million monetizable daily active users as of July last year, according to the company's last public disclosure before Musk's takeover, although data from web analytics companies indicates usage has dropped since then.

Twitter's web traffic was down 11 percent from the year prior in the days after the Threads launch, compared to the 4 percent it was down year-over-year as of June, according to Similarweb.

Matthew Prince, CEO of internet infrastructure firm Cloudflare, shared a graph showing a similar trajectory in a tweet on Sunday and said Twitter's traffic was "tanking."

Twitter CEO Linda Yaccarino tweeted on Monday that the platform last week had its "largest usage day" since February, without providing details. "There's only ONE Twitter," she tweeted.

Musk has responded to Threads' arrival by mocking it and threatening to sue Meta, alleging that the social media behemoth used its trade secrets and other confidential information to build the app.

That claim, legal experts say, could be hard to prove.

Threads, like other would-be rivals, bears a strong resemblance to Twitter. It allows posts that are up to 500 characters long and supports links, photos and videos of up to five minutes.

The app also does not yet have a direct messaging function and lacks a desktop version that certain users, such as business organizations, rely on.

It also currently lacks hashtags and keyword search functions, which limits its appeal to advertisers and its utility as a place for following real-time events like what users frequently do on Twitter.

Still, analysts said the turmoil at Twitter, including uproar over recently imposed limits on the number of tweets users can see, could help Threads to attract those groups.

Currently, there are no ads on the Threads app and Zuckerberg said the company would only think about monetization once there was a clear path to one billion users.

Instagram head Adam Mosseri said last week Meta was not trying to replace Twitter and that Threads aimed to focus on light subjects like sports, music, fashion and design.

He acknowledged that politics and hard news are inevitably going to show up on Threads, in what would be a challenge for the app pitching itself as the "friendly" option for public discourse online.

(With input from Reuters)