More than 660,000 seniors next year will lose the private Medicare plans they now have because some insurers are dropping coverage in response to tougher federal requirements. Most of those beneficiaries are enrolled in a type of Medicare Advantage plan called Private Fee for Service, where enrollment has surged from about 820,000 three years ago to more than 2.44 million. The high cost of PFFS plans led Congress to vote in 2008 to require the plans to establish networks of providers beginning in 2011.