Twitter shares surged nearly 28 percent on Monday after regulatory filings in the US revealed that Tesla and SpaceX CEO Elon Musk has a 9.2 percent passive stake in Parag Agrawal-run platform.

According to a regulatory filing in the US Securities and Exchange Commission (SEC) in the US, the world’s richest man, with over 80 million followers on Twitter, bought nearly 73.5 million shares in the micro-blogging platform.

Last month, Musk said he is considering building a new social media platform.

“Am giving serious thought to this,” he wrote on the micro-blogging site in a reply to a user.

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Musk has been a vocal critic of social media platforms, including Twitter. In a Twitter poll, Musk had asked: “do you believe Twitter rigorously adheres to this principle?” and at least “70 percent” of respondents said “no”.

“Given that Twitter serves as the de facto public town square, failing to adhere to free speech principles fundamentally undermines democracy. What should be done?” Musk wrote in a response to the poll result. He also asked users “is a new platform needed?”

Musk’s legal skirmish with the US SEC also continues. Musk is currently looking to throw out his 2018 agreement with the SEC, which requires him to get pre-approval for tweets that are of material importance to Tesla.

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