Check out the companies making headlines in midday trading.

Twitter — Shares of the social media company dropped more than 8% after Elon Musk walked away from his $44 billion deal to buy Twitter. Musk alleged that Twitter under-reported the number of spam bots on the platform. The two parties are likely set for a protracted court battle, and Musk could also be faced with paying a $1 billion breakup fee.

Casino stocks — Shares of Wynn Resorts and Las Vegas Sands dove 9.4% and 8.8%, respectively, after Macao ushered in a week-long shutdown as it grapples with a Covid-19 outbreak. Monday marked the first time in more than two years that Macao has shut down all of its casinos.

Lululemon, Under Armour — Shares of the activewear retailers were lower following downgrades by Jefferies. Lululemon fell 4% after the firm lowered its rating on the stock to underperform from hold, citing "rising competition." Under Armour declined by some 4.7%. Jefferies downgraded it to neutral from buy, saying fundamentals are "lagging."

Meta Platforms — The social media company's stock dropped 4.2% after Needham downgraded it to underperform from hold. The firm pointed to Meta's heavy investments into the metaverse, which may take too long to pay off.

Uber — The ridesharing stock fell more than 4% following a report by the International Consortium of Investigative Journalists that said Uber has lobbied extensively to relax labor and tax laws and used "stealth technology" to block government scrutiny. The company issued a statement acknowledging prior mistakes and emphasizing Uber "is a different company today."

Nio — Nio shares slid 8.4% as China appears to be battling another wave of Covid-19. Reuters reported that multiple Chinese cities have imposed new health restrictions. The automaker also announced that it has formed a committee to investigate allegations made against Nio by a short-seller last month.

Amazon — The ecommerce giant lost 2.3% after Bloomberg reported that the number of U.S. Prime customers stalled in the first half of the year, possibly in part because of the $20 membership price hike that took place in February. Amazon had 172 million members on June 30, level with six months prior, the report said, citing Consumer Intelligence Research Partners.

Upstart — Upstart jumped as much as 2.6% Monday as investors looked to buy the dip. The company's stock took a hit last week after it announced it would not meet its already-reduced financial targets for the second quarter and JMP Securities downgraded it. Shares are down more than 80% this year.

 — CNBC's Yun Li, Sarah Min, Samantha Subin, Carmen Reinicke and Jesse Pound contributed reporting.