Elon Musk helped Twitter have its best day since IPO.

• Since its IPO, Twitter had its best day after Elon Musk, CEO of Tesla and SpaceX, acquired a 9.2% passive stake in the company.
• Shares closed up more than 27%.
• Nearly 73% of the company's stock was closed after its public debut on Nov. 7, 2013.

On Monday, Twitter's stock had its best day since its IPO, with shares up more than 27%, after an SEC filing disclosed that Elon Musk had acquired a 9.2% passive stake in the social media company.

Nearly 73% of Twitter's stock was closed after its public debut on Nov. 7, 2013. Despite being down 38% from its all-time high in February 2021, the stock is still trading.

Some investors anticipate the outspoken executive will adopt an aggressive approach, such as boosting his share or seizing control of the company to drive change.

According to Truist Securities' analysts, given that the investment is only a small fraction (~1%) of Musk's net worth, it is not surprising that he may increase his stake and potentially take a more active role in the company's decision-making.

Twitter declined to comment.

The CEO's investment in a new social media platform comes after he expressed dissatisfaction with the scrutiny of his tweets by regulators such as the U.S. Securities and Exchange Commission.

Twitter's role as a public town square means that upholding free speech principles is crucial for democracy. What action should be taken?

In November, the SEC subpoenaed Musk after he requested his Twitter followers' opinion on whether he should sell 10% of his Tesla stock, resulting in a decline in share prices.

In February, Musk accused the SEC of "harassment" and attempting to "suppress" his right to free speech.

— CNBC’s Sam Shead contributed to this report.