Not to speak for GP, but that is a perfectly reasonable position to take, and one that I would take. They're different services; GMail does not have third-party

Not to speak for GP, but that is a perfectly reasonable position to take, and one that I would take. They're different services; GMail does not have third-party apps in the same sense. Facebook's app ecosystem has some quite disreputable parties in it; an API that lets these parties have access to friends' email addresses would place a lot of Facebook users' emails in the hands of companies that they have no direct relationship with or knowledge of.