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.

Much like Twitter, the app features short text posts that users can like, re-post and reply to, although it does not include any direct message capabilities. Posts can be up to 500 characters long and include links, photos and videos up to five minutes long, according to a Meta blog post.

It is available in more than 100 countries on both Apple's App Store and Google's Play Store, the blog post said.

Analysts said investors were salivating over the possibility that Threads' ties to Instagram might give it a built-in user base and advertising apparatus. That could siphon ad dollars from Twitter at a time when the microblogging company's new CEO is trying to revive its struggling business, Reuters reported.

While Threads launched as a standalone app, users can log in using their Instagram credentials and follow the same accounts, potentially making it an easy addition to existing habits for Instagram's more than 2 billion monthly active users.

"Investors can't help but be a little excited about the prospect that Meta really has a 'Twitter-Killer'," said Danni Hewson, head of financial analysis at investment firm AJ Bell.

Meta stock closed up 3% on Wednesday ahead of the launch, outpacing gains by competitor tech companies as the broader market edged down.

Threads' arrival comes after Zuckerberg and Musk have traded barbs for months and even threatened to fight each other in a real-life mixed martial arts cage match in Las Vegas.

The timing is opportune for Meta to land a blow, as months of Musk's chaotic decision-making has roiled Twitter.

Musk bought Twitter for $44 billion last October, but its value has since plummeted as it faced an exodus of advertisers amid deep staffing cuts and content moderation controversies. Its latest move involved limiting the number of tweets users can read per day.

Zuckerberg, in subsequent Threads posts, addressed those challenges. "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," he wrote.

The integration with Instagram included several nods to privacy considerations. Instagram users who sign up for Threads automatically have a badge affixed to their Instagram profile, but can opt to hide it. They also are given options to choose different privacy settings for each app.

Brands like Billboard, HBO, NPR and Netflix had accounts set up within minutes of launch, as did celebrities like Shakira and other well-known personalities such as former Meta Chief Operating Officer Sheryl Sandberg. The app did not appear to show any ads, according to a Reuters review.

To build up Threads, Meta has been making overtures to social media influencers to attract them to the new app and encouraging them to post at least twice a day, said Ryan Detert, CEO of influencer marketing company Influential.

Some thanked the company for early access in their initial posts.

The app also benefits from the failure of other would-be Twitter competitors to take advantage of the service's stumbles. While a number of burgeoning competitors such as Mastodon, Post, Truth Social and T2 have tried to lure Twitter users away, all remain relatively small so far.

Bluesky, a new service backed by Twitter cofounder Jack Dorsey, launched its invite-only beta in February and initially had users clamoring to get access codes. Its website said it had 50,000 users as of April. Dorsey also backed another platform called Nostr.