Elon Musk, who runs four other companies, will now also be Twitter CEO

Tesla Inc boss Elon Musk said in a filing on Monday he will serve as chief executive of Twitter, the social media company he just bought for $44 billion, a move that Wall Street analysts have said could stretch the billionaire thin.

Musk, who also runs rocket company SpaceX, brain-chip startup Neuralink and tunneling firm the Boring Company, fired Twitter’s previous chief, Parag Agrawal, and other top company officials last week.

Tesla’s stock has lost a third of its value since Musk made an offer to buy Twitter in April, compared with a 12% decline in the benchmark S&P 500 index in the same period.

Musk had previously changed his Twitter bio to “Chief Twit” in an allusion to his planned move.