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

The Facebook parent’s CEO Mark Zuckerberg said this Thursday, as the app went live on Apple and Android app stores in 100 countries, and will run with no ads for now.

Threads is the biggest challenger yet to Elon Musk-owned Twitter, which has seen a series of potential competitors emerge but not yet replace it.

Meanwhile, Twitter threatened to sue Meta just hours after the Instagram parent company launched Threads. In a letter to Meta CEO Mark Zuckerberg, Musk’s lawyer Alex Spiro accused the company of hiring dozens of former Twitter employees who “had and continue to have access to Twitter’s trade secrets and other highly confidential information.” The letter accused Meta of “unlawful misappropriation of Twitter’s trade secrets and other intellectual property”