From By Fred Schulte , Maia Rosenfeld , David Hilzenrath Medicare Advantage Majority has spent more than $3.1 million on hundreds of Facebook ads since September 2024, according to Facebook's Ad Library, a database of the company's online ads. (Eric Harkleroad/KFF Health News) Eric Harkleroad/KFF Health News hide caption Judging by more than 16,300 comments recently posted on a federal government website, you'd think there was a groundswell of older Americans demanding that federal officials hike payments to their Medicare Advantage health insurance plans. Yet about 83% of the comments are identical to a letter that appeared on the website of a secretive advocacy group called Medicare Advantage Majority, a data analysis by KFF Health News has found.…