Investors on Monday cheered Elon Musk’s reveal that he has become Twitter’s largest shareholder, sending the stock up 27.1% and adding almost a billion dollars to Musk’s net worth.

Musk has taken a 9.2% stake in Twitter, some 73.5 million shares, according to a new SEC filing. The investment is worth $3.7 billion today, up from $2.9 billion at the end of the previous trading day. The bulk of Musk’s fortune is in Tesla shares, and with that stock rapidly appreciating in value over the last two years, Musk is now the world’s richest person, worth over $300 billion.

Musk has suggested on Twitter that he’d like to see some changes made to Twitter, though it’s unclear exactly for what he wants and how he’ll use his large shareholding to cajole for change. That said, it is clear he’s unhappy with the company’s direction, criticizing it in recent posts for its moderation policies, which conservatives believe limit free speech.

Musk’s complaints amount to something of an indictment of Jack Dorsey’s time in the CEO role, a position he turned over to Parag Agrawal in November. Nevertheless, Musk’s appearance on the scene raised Dorsey’s personal fortune on Monday, too. Dorsey still holds a 2.25% stake in the company, a position almost $200 million more valuable after Musk unveiled his stake. Another billionaire Twitter cofounder, Ev Williams, saw his fortune increase by $90 million.Follow me on Twitter. 

By Abram Brown, Forbes Staff