Tesla top boss Elon Musk revealed a 9.2 per cent stake in Twitter, worth nearly US$3 billion, making him the micro-blogging site's largest shareholder and triggering a more than 25 per cent rise in the company's shares.
Musk's move comes close on the heels of his tweet that he was giving a "serious thought" to building a new social media platform, while questioning Twitter's commitment to free speech.
A prolific Twitter user, Musk has over 80 million followers since joining the site in 2009 and has used the platform to make several announcements, including teasing a go-private deal for Tesla that landed him in regulatory crosshairs.
Of late, however, the world's richest person has been critical of the social media platform and its policies, and recently ran a Twitter poll asking users if they believed the platform adheres to the principle of free speech, to which over 70 per cent voted "no".
Twitter's recent quarterly results and lower-than-expected user additions have raised questions about its growth prospects, even as it pursues big projects such as audio chat rooms and newsletters to end long-running stagnation.
"It does send a message to Twitter ... having a meaningful stake in the company will keep them on their toes, because that passive stake could very quickly become an active stake," said Thomas Hayes, managing member at Great Hill Capital LLC.
Musk - who, according to Forbes, has a net worth of about US$300 billion - has been selling his stake in Tesla since November, when he said he would offload 10 per cent of his holding in the electric-car maker. He has already sold US$16.4 billion worth of shares since then.
A regulatory filing on Monday showed that Musk owns 73.5 million Twitter shares, which are held by the Elon Musk Revocable Trust, of which he is the sole trustee. Vanguard is Twitter's second-biggest shareholder, with an 8.79 per cent stake, according to Refinitiv data.
"Musk's actual investment is a very small percentage of his wealth and an all-out buyout should not be ruled out," CFRA Research analyst Angelo Zino wrote in a client note.
Musk has previously made early-stage investments in companies, including online payment processor Stripe Inc and artificial intelligence firm Vicarious, but his latest investment would be the first of its kind.
He is also the founder and chief executive officer of SpaceX, and leads brain-chip startup Neuralink and infrastructure firm The Boring Company.
Twitter was the target of activist investor Elliott Management Corp in 2020, when the hedge fund argued its then-boss and co-founder, Jack Dorsey, was paying too little attention to Twitter while also running Square.
Dorsey, who owns a more than 2 per cent stake in Twitter, stepped down as CEO and chairman in November last year, handing over the reins to company veteran Parag Agrawal.
Agrawal reportedly went on a paternal leave for a few weeks in February.
Meanwhile, Musk and Dorsey have found some common ground in dismissing the so-called Web3, a vague term for a utopian version of the internet that is decentralised.
Twitter shares, which had fallen 38 per cent in the past 12 months through Friday close, were trading at US$47.19. The company added as much as US$8.38 billion to its market capitalisation, which now stands at US$39.3 billion.
Twitter did not respond to a Reuters request for comment.