Not sure if this is the right answer, but a guess: it's easy for other web services to access this stored data if the user provides them with their Facebook login and password, which no self-respecting service would ever ask for. Even if Google did at one point get users to give up their facebook credentials, the API's they access will probably block IP's that try to access multiple users' data in a short amount of time.

The best solution, IMO, is to use the browser as a platform: a Chrome extension can allow users to seamlessly sync data between services, because Chrome already has access both to your Facebook account and your Picasa account.