With Twitter already on the ropes, Meta's Mark Zuckerberg delivered another blow to Elon Musk on Wednesday, ramping up the tech billionaires' rivalry with the launch of Instagram's much-anticipated companion service Threads, a challenger to Twitter. "Let's do this. Welcome to Threads," Zuckerberg wrote in his first post on the app, along with a fire emoji. He said the app logged 5 million sign-ups in its first four hours.