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, 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."

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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 discussed a head-to-head cage fight on social media.

Musk meanwhile retweeted an image that said the Threads logo resembled a tapeworm. "Metaphorically too," he added. In another post referencing Twitter's potential legal action against Meta, Musk noted that "competition is fine, cheating is not."

Since Musk's $44 billion takeover of Twitter, he has made around 50 percent of its 7,500-strong global workforce redundant and there has been accusations some have ended up on the Threads project. Meta spokesman Andy Stone said on Threads: "No one on the Threads engineering team is a former Twitter employee - that's just not a thing."

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.

"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.

Accounts were already active for celebrities such as Jennifer Lopez, Shakira, Oprah Winfrey and Hugh Jackman, as well as media outlets including The Washington Post and The Economist.

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 had the opportunity to do this but hasn't nailed it. Hopefully we will."

Twitter has said it has more than 200 million daily users and 1.6 billion accounts in active use.

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