Novartis also failed to notify the FDA of the Tasigna promotion, as required by the agency, the letter said.

Novartis, in a statement, said it takes the FDA letter "very seriously."

"We have addressed its concerns by taking the direct and immediate action of taking down the widget referenced by the FDA," the company said. "Novartis will continue to have active discussions with the FDA to understand fully all of the concerns. We also will assess all of our Web assets and materials based on these concerns."

The warning letter highlights a problem with Facebook and other websites targeting advertising dollars from pharmaceutical and companies, said Chester, who asked the FDA in March to of drugs. Facebook should be responsible for creating safeguards for the marketing of medications and other health products on its site, he said.

Websites "see digital dollars in their bottom lines and are ignoring the health risks confronting consumers who are targeted using social-media marketing," Chester said. "This letter is a wake-up call -- for the pharma industry, Facebook, and social media application companies."