Tesla boss Elon Musk revealed a 9.2% stake in Twitter today, likely making him the biggest shareholder in the micro-blogging site and triggering a 23% rise in its 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 filing showed that Musk owns 73.5 million Twitter shares, valuing his passive stake in the company at up to $2.9 billion based on the stock's Friday close.

The shares are held by the Elon Musk Revocable Trust, of which he is the sole trustee.

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 scrutiny.

Of late, however, he has been critical of the social media platform and its policies, saying the company is undermining democracy by failing to adhere to free-speech principles.

Musk has been selling his stake in Tesla since November, when he said he would offload 10% of his holding in the electric-car maker. He has already sold $16.4 billion worth of shares since then.

Twitter was the target of activist investor Elliott Management 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 stepped down as CEO and chairman in November last year, but continues to own a 2.25% stake in the company that makes him the sixth biggest shareholder, according to Refinitiv data.

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.

Shares of other social media firms, including Meta Platforms and Snapchat owner Snap, were also trading higher on the news.