Twitter was once a necessity for major brands. Under Elon Musk, it's now "high risk."

Twitter was once the place where major brands promoted new products, responded to customer complaints and honed their image. But after less than three weeks of ownership under Elon Musk, the social media service is now viewed as "high risk" by one major advertising company — and other big brands are continuing to bail.

Luxury fashion brand Balenciaga deleted its Twitter account this week, the latest high-profile company to retreat from the social media service amid a litany of problems under Musk's leadership. Eli Lilly has withdrawn its advertising from Twitter after a fake account tweeted that it was making its insulin products free, according to the Washington Post. That fake tweet caused Eli Lilly's stock to drop and placed renewed focus on the high price of insulin.

And the world's biggest media buyer, Group M, is now warning its clients that advertising on Twitter is "high risk," officials at Group M confirmed. Group M works with brands like Google, Ford and Coca-Cola on buying ads across media services, representing 1 in every 3 ads globally.