Twitter threatened to sue Meta just hours after the Instagram parent company launched Threads, an app it hopes will beat out the struggling site owned by Elon Musk.
In a letter to Meta CEO Mark Zuckerberg, published by online news outlet Semafor on Thursday, Musk lawyer Alex Spiro accused the company of "unlawful misappropriation of Twitter's trade secrets and other intellectual property."
The letter accused Meta of hiring dozens of former Twitter employees who "had and continue to have access to Twitter's trade secrets and other highly confidential information."
Threads is the biggest challenger yet to Musk-owned Twitter, which has seen a series of potential competitors emerge but not yet replace one of the world's biggest social media platforms, despite its struggles.
Zuckerberg's latest move against Musk further heightened the rivalry between the two multibillionaires who have even agreed to meet for hand to hand combat in a cage match.
And in his first tweet in over a decade, Zuckerberg posted a Spiderman pointing at Spiderman meme in an apparent reference to the similarities between Threads and Twitter.
Threads went live on Apple and Android app stores in 100 countries at 2300 GMT on Wednesday, and early feedback noted its close, but scaled back, resemblance to Twitter.
Within a few hours, more than 30 million people had downloaded Threads, Zuckerberg said Thursday.
"Feels like the beginning of something special, but we've got a lot of work ahead to build the app," Zuckerberg wrote on his official Threads account Thursday.
Accounts were already active for celebrities such as Jennifer Lopez, Shakira and Hugh Jackman, as well as media outlets including The Washington Post and The Economist.
On Threads, Zuckerberg wrote: "It'll take some time, but I think there should be a public conversations app with 1 billion+ people on it."
Twitter has said it has more than 200 million daily users.
Threads was introduced as a spin-off of Instagram, giving it a built-in audience of more than two billion users and sparing the new platform the challenge of starting from scratch.
Instagram chief Adam Mosseri told users that Threads was intended to build "an open and friendly platform for conversations."
"The best thing you can do if you want that too is be kind," he said.
Zuckerberg is taking advantage of Musk's chaotic ownership of Twitter to push out the new product, which Meta hopes will become the go-to platform for celebrities, companies and politicians.
Analyst Jasmine Engberg from Insider Intelligence said Threads only needs one out of four Instagram monthly users "to make it as big as Twitter."
"Twitter users are desperate for an alternative, and Musk has given Zuckerberg an opening," she added.
Under Musk, Twitter has seen content moderation reduced to a minimum with glitches and rash decisions scaring away celebrities and major advertisers.
He also fired more than half of Twitter's staff, some of whom presumably went to other tech companies, including Meta.
Meta has its legion of critics too, especially in the major market of Europe, which could slow the growth of Threads.
The company has been criticised for its handling of personal data, the essential ingredient for targeted ads that help it rake in billions of dollars in profits.
Mosseri said he regretted that the launch was delayed in the European Union, but had Meta waited for regulatory clarity from Brussels, Threads would have been "many, many, many, months away."
According to a source close to the matter, Meta was wary of a new law called the Digital Markets Act (DMA), which sets strict rules for the world's "gatekeeper" internet companies.
One rule restricts platforms from moving user data between products, as would potentially be the case between Threads and Instagram.
Globally, the Threads hashtag on Twitter has garnered three million tweets, with many users jokingly suggesting people will return to Musk's platform.
Others expressed privacy concerns.
"Meta loves to collect private information and I don't trust the way it treats private information," a Japanese user tweeted.
"I also have the impression that this is a company hated by EU, so I'm reluctant."
But some said they would permanently move to Threads.
One Threads user wrote: "Now I truly can say goodbye to Twitter forever."