Elon Musk, who also owns Tesla and SpaceX, has become one of the biggest shareholders in Twitter, just a week after the business tycoon posted on the platform saying he was giving “serious thought” to building a new social media company. A filing disclosed on Monday by the US Securities and Exchange Commission (SEC) revealed that the South African billionaire had taken a passive 9.2 per cent stake worth $2.9bn. The world’s richest man worth an estimated $273billion bought nearly 73.5 million shares, a document filed with the US Securities and Exchange Commission shows. It means the Tesla co-founder has more than quadruple the shares of Twitter’s founder Jack Dorsey, who owns 2.25 per cent. The news prompted Twitter’s stock to soar by more than 26 per cent in pre-market trade before Wall Street opened on Monday, meaning his shares are now worth around $3.6billion, earning him an interested $1.2billion in three weeks. 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. Elon 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.