Linda Yaccarino, a former NBCUniversal marketing executive, on Monday officially took over the CEO role at Twitter from Elon Musk, weeks after the billionaire announced hiring her for the top spot at the social media company. “It happened — first day in the books!,” Yaccarino tweeted late Monday. “Stay tuned…” A day earlier, Yaccarino had tweeted that Twitter had hired Joe Benarroch, NBCU’s former senior vice president of communications for advertising and partnerships, to join her at the company. Benarroch will work in business operations, according to his LinkedIn profile. “Let’s get to work @Twitter! #timetofly” Yaccarino said in the tweet. The New York Times and The Information first reported that Yaccarino was expected to officially start as Twitter CEO on Monday. Twitter did not respond to a request for comment. Musk announced in May that Yaccarino, formerly chairman of global advertising and partnerships at NBCU, would be taking over as CEO of Twitter with a focus on business operations, after months of suggesting that he planned to find a new top leader for the struggling platform. In December, Musk ran a poll on the platform asking users whether he should step back as Twitter’s CEO, which ended with the majority of users voting in the affirmative. A bet on reviving Twitter’s ad business Musk is likely betting on Yaccarino’s ad industry chops to restore Twitter’s flagging advertising business. On Monday, the New York Times reported that Twitter’s US advertising revenue from the five weeks from April 1 to the first week of May was down 59% year-over-year, citing an internal presentation. (CNN has not viewed the presentation.) Just 43% of Twitter’s top 1,000 advertisers as of September, the month before Musk’s takeover, were still advertising on the platform as of April, according to data provided to CNN by market intelligence firm Sensor Tower last month. Reversing that decline could prove to be an uphill battle: Twitter’s ad business has declined sharply since Musk’s takeover due to advertiser concerns over hate speech on the platform, mass layoffs at the company and general uncertainty about Twitter’s future direction. Yaccarino’s takeover comes just days after the resignation of Twitter’s head of trust and safety, Ella Irwin, who confirmed her exit in a tweet Friday. Yaccarino may also struggle to rein in Musk, whose tweets and policy decisions often generate controversy. Musk last month told CNBC he “didn’t care” if his controversial tweets drew the ire of Twitter advertisers or Tesla shareholders. “I’ll say what I want to say, and if the consequence of that is losing money, so be it,” he said. Even without the CEO title, Musk will retain significant control over Twitter as its owner. He has said he will oversee product, technology and software and systems operations, and will also transition to being the company’s executive chairman. Fighting to break even Yaccarino’s first day coincided with a Twitter Spaces livestream event Musk hosted with Democratic presidential hopeful and anti-vaccine advocate Robert F. Kennedy, Jr, after a similar event Musk held with Ron DeSantis last month was largely overshadowed by technical glitches. At the event, Musk lamented Twitter’s fraught relationship with advertisers. Musk told Kennedy that Twitter had lost billions of dollars in advertising revenue due to Musk’s approach to content moderation on the social platform. “It’s definitely been extremely difficult,” Musk said. “Basically, our revenue is cut in half because we didn’t toe the line.” The Spaces conversation, which drew more than 60,000 listeners at its peak, was hosted by Musk, Kennedy and tech investor David Sacks. Musk discussed the financial toll advertising boycotts have had on his business, saying it’s been a “huge struggle for Twitter to break even.” “We’re hoping to break even, but we’re not there yet,” he said.