Elon Musk is no longer the CEO of Twitter

• Elon Musk announced last month he'd found a replacement for the Twitter CEO role.
• Linda Yaccarino, a former NBCU ad sales chief, said she started the job Monday.
• She began two weeks ahead of schedule, with Twitter's ad business currently struggling.

Elon Musk is no longer the CEO of Twitter, as Linda Yaccarino started at the company on Monday.

Around 8 p.m. PT, the former NBCU ad sales chief tweeted: "It happened — first day in the books!"

Yaccarino was originally slated to start around June 22, since Musk tweeted May 11 that she would start in six weeks. But The Information first reported that she would instead take the reins more than two weeks ahead of schedule.

"My role will transition to being exec chair & CTO, overseeing product, software & sysops," Musk said.

He'd been in the role since buying Twitter for $44 billion last October. According to one stakeholder, Fidelity, the company is now worth one-third of that.

Musk first said he would step down as Twitter CEO in December after users voted in a poll.

Although as recently as April, he was instead joking that his dog was now the boss.

Yaccarino assumes the tough task of making Twitter profitable and revitalizing its advertising business.

According to an internal presentation obtained by The New York Times, Twitter's US ad revenue is down 59% compared to the same period last year - and it's regularly falling short of weekly projections by as much as 30%.

Insider reported Monday that Musk is prioritizing Twitter's live video service. Some advertisers speculate that he wants to make the platform video-centric, which would make sense given Yaccarino's expertise in selling ads alongside video content.

Yaccarino's prompt start follows the resignations of two top execs in Twitter's safety division.

Musk said they quit in relation to controversy over Twitter limiting the visibility of "What is a Woman?" - a documentary about transgender people made by conservative commentator Matt Walsh.

Insider contacted Twitter for comment. The company responded with an automated message that didn't address the inquiry.