Over 10 million people have signed up to Threads, Meta’s rival to Twitter, within the first few hours of its launch, announced Meta’s CEO Mark Zuckerberg on Thursday.

The app has been made available on Apple and Android app stores in 100 countries and will initially run without ads.

Threads poses a significant challenge to Twitter, which is owned by Elon Musk. Despite several emerging potential competitors, none have managed to replace Twitter as effectively as Threads aims to do.

Shortly after Meta launched Threads, Twitter responded by threatening to sue the company.

In a letter addressed to Meta CEO Mark Zuckerberg, Elon Musk’s lawyer, Alex Spiro, accused Meta of hiring numerous former Twitter employees who possess “access to Twitter’s trade secrets and other highly confidential information.” The letter accused Meta of engaging in the “unlawful misappropriation of Twitter’s trade secrets and other intellectual property.”