Elon Musk has bought a 9.2% stake in Twitter, according to a regulatory filing published Monday by the Securities and Exchange Commission.

Why it matters: Musk recently told his around 80 million Twitter followers he's giving "serious thought" to building a social media platform to rival Twitter, which he criticized for "failing to adhere to free speech principles."

By the numbers: Twitter shares jumped more than 25% in premarket trading after news of Musk's purchase broke.

• Musk's 9.2% stake is equivalent to 73,486,938 shares of Twitter common stock and is worth $2.89 billion based on the company's closing price Friday.
• The stake would make Musk the company's largest shareholder, per the Financial Times. Twitter co-founder Jack Dorsey holds a 2.25% stake.

Go deeper ... The new silent majority: People who don't tweet