The swell of enthusiasm around the new app Threads seems to be too much for Elon Musk.

The CEO of Twitter has been lashing out at fellow tech mogul Mark Zuckerberg since the release of Meta’s micro-blogging app last Tuesday, but he truly aimed below the belt when he challenged his rival to “a literal dick-measuring contest” on Sunday night.

Musk’s proposition followed a crass jab he tweeted hours earlier, “Zuck is a cuck” ― short for “cuckold”, which has become a go-to insult for the right-wing set.

The embattled tech CEO has already threatened to sue Meta over Threads, which TechCrunch reports hit 100 million users by Monday morning, just five days after its launch.

Twitter currently has around 250 million active users, but data from internet services company Cloudflare seems to show traffic has been steadily declining since the start of 2023, following Musk’s purchase of the app for $44 billion last fall.

Musk’s feud with Zuckerberg took a decidedly personal turn last month after he challenged the Facebook founder to a cage match. In a surprising twist, the usually mild Zuckerberg tweeted back at Musk, writing, “Send me location.”

Zuckerberg has pitched Threads as a more welcoming alternative to Twitter and its quarrelsome culture.

“The goal is to keep it friendly as it expands,” he wrote on Threads last week. “I think it’s possible and will ultimately be the key to its success. That’s one reason why Twitter never succeeded as much as I think it should have, and we want to do it differently.”