By Katie Paul

NEW YORK (Reuters) -Meta's Mark Zuckerberg delivered a blow to Elon Musk on Wednesday night, as the tech billionaires' rivalry went live with the launch of Instagram's much-anticipated Threads platform, a clone of Twitter.

"Let's do this. Welcome to Threads," Zuckerberg posted on the app, along with a fire emoji.

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, whose new CEO is trying to revive the microblogging company's struggling business.

While Threads launched as a standalone app, users could 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 platform 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.

While Meta is likely to focus first on growing users before incorporating advertising on Threads, big brands "will happily (invest) a good amount of ad spend on the platform" to seek refuge in its relative brand safety and capitalize on early buzz, said Matt Navarra, a social media consultant who has worked with Meta, Google and Pinterest.

"We're here to foster a positive and creative space for you to express your ideas. This time, it's all about conversation," said a post by the official Threads account.

Brands like Billboard, HBO and Variety 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 new and 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 instantly created a buzz on Twitter, with users clamoring to get access codes. Its website says it has 50,000 users. Dorsey has also backed another platform called Nostr.

But history is working against Meta. It has 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 directly into Instagram and more recently wound down its unit tasked with designing experimental apps as part of a cost-cutting drive.

Another potential strike against Threads is that the news-oriented culture on Twitter is different from that on Instagram, a more visual platform, said Jasmine Enberg, principal analyst at Insider Intelligence.

"The main use cases for Twitter still remain keeping up with news and world events," said Enberg. "I find it hard to imagine that the most avid loyal Twitter users who go to Twitter for that type of culture will defect and go immediately to Threads."

Still, she said, Meta only needs to convince a quarter of Instagram's users to join Threads in order to rival Twitter's size. "The reality is that Meta doesn't need to convert Twitter power users into Threads users."

(Reporting by Katie Paul; Additional reporting by Sheila Dang, Chavi Mehta and Bansari Kamdar; Editing by David Gregorio and Christopher Cushing)