Meta Platforms (META.O) on Wednesday launched a direct challenge to Twitter with Threads, garnering millions of users in hours as it sought to take advantage of its rival's much-weakened state after a series of chaotic decisions from owner Elon Musk.

Those quick to join the new platform included celebrities such as Kim Kardashian and Jennifer Lopez as well as prominent politicians such as Democratic US Representative Alexandria Ocasio-Cortez.

"Let's do this. Welcome to Threads," Meta CEO Mark Zuckerberg wrote in his first post on the app, along with a fire emoji. He said the app logged 10 million sign-ups in seven hours.

He also took to Twitter, posting a well-known meme of Spiderman facing off against Spiderman - in a humorous jab at the rivalry with Musk and between the two services.

Analysts have said 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 its new CEO is trying to revive its struggling business.

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.

Others saw the launch of Threads as an opportunity to create a less toxic version of Twitter.

"May this platform have good vibes, strong community, excellent humor, and less harassment," Ocasio-Cortez said in her post.

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.

Meta stock closed up 3% on Wednesday ahead of the launch, outpacing gains by rival tech firms.

Twitter on the back foot

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

Meta is taking aim at a time when Twitter is definitely on the defensive.

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

Zuckerberg noted the challenges that big public social media forums bring. "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. 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.

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.

Meta has, however, suffered multiple failures launching standalone copycat apps in the past, most notably its Lasso app aimed at competing with short video rival TikTok.

The company later incorporated a short video tool, Reels, directly into Instagram and more recently wound down its unit tasked with designing experimental apps.

Zuckerberg, responding to a user who predicted Twitter's demise about an hour after the Threads launch, cautioned patience. "We're only in the opening moments of the first round here," he said.