Meta's Threads racked up more than 30 million sign-ups within about 18 hours of its launch, emerging as the first real threat to Elon Musk-owned Twitter, as it took advantage of its access to billions of Instagram users and a similar look to that of its rival.

Dubbed as the "Twitter-Killer", Threads was the top free app on Apple's App Store in the UK and the U.S. on Thursday. Its arrival comes after Meta CEO Mark Zuckerberg and Twitter's Musk have traded barbs for months, even threatening to fight each other in a real-life mixed martial arts cage match in Las Vegas.

"The cage match has started, and Zuckerberg delivered a major blow. In many ways, it's exactly what you'd expect from Meta: Stellar execution and an easy-to-navigate user interface," Insider Intelligence principal analyst Jasmine Enberg said.

Twitter responded on Thursday by threatening to sue Meta, according to the publication Semafor, citing a letter delivered to Zuckerberg by a lawyer for Twitter. Twitter was not immediately available for comment. Reuters could not immediately verify the Semafor report.

Meta said no one on the Threads engineering team is a former Twitter employee.

Numerous competitors to Twitter have sprung up following Musk's $44 billion purchase of the social media platform last year, which was followed by a series of chaotic decisions that have alienated both users and advertisers. Musk's latest move involved limiting the number of tweets users can read per day.

Twitter's stumbles make room for a well-funded competitor like Meta Platforms, analysts and experts said, particularly because of its access to Instagram users and its advertising strength.

"Meta's release of Threads came at the perfect time to give it a fighting chance to unseat Twitter," said Niklas Myhr, professor of marketing at Chapman University, referring to the turmoil at Twitter after it limited the number of tweets users can see.

"Threads will be off to a running start as it is built upon the Instagram platform with its massive user base and if users adopt Threads, advertisers will be following closely behind."

Other competitors have found limited success. Mastodon, another Twitter-like app, has 1.7 million monthly active users, according to its website, while Twitter co-founder Jack Dorsey-backed Bluesky has about 265,000 users.

Twitter had 229 million monthly active users in May 2022, according to a statement made before Musk's buyout.

Threads has certain limitations

While Threads is a standalone app, users can log in using their Instagram credentials, which makes it an easy addition for Instagram's more than 2 billion monthly active users.

Threads' launch was clearly a first stab at a service as it currently lacks the bells and whistles of Twitter.

"There should be a public conversations app with 1 billion+ people on it. Twitter has had the opportunity to do this but hasn't nailed it. Hopefully we will," Zuckerberg said on Threads, where he now has a million followers.

Threads does not have hashtags and keyword search functions, which means users cannot follow real-time events like on Twitter. It also does not yet have a direct messaging function and lacks a desktop version that certain users, such as business organizations, rely on.

Some users including tech reviewer Marques Brownlee posted about the need for a feed that only consists of the people one follows. Users currently have little control over the main feed.

Twitter CEO Linda Yaccarino, who was hired by Musk in May to shore up advertiser confidence, said in tweet on Thursday that "everyone's voice matters" on the app. "We're often imitated -- but the Twitter community can never be duplicated."

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 1 billion users.

Existing ad relationships from Instagram and Facebook should help Threads' revenue, said Pinar Yildirim, associate professor of marketing at the University of Pennsylvania's Wharton School.

"Facebook is a less uncertain bet compared to Twitter and a bigger player in the ad market."

Some analysts said Threads was reminiscent of Meta's success in integrating crucial features of platforms such as Snapchat and TikTok in the case of Instagram's Stories and Reels.

At least four brokerages raised their price target on Meta, whose shares have already more than doubled in value this year.

On Thursday, Meta shares were down 0.2 per cent, after rising 3 per cent on Wednesday ahead of Threads' launch.