But Google/Facebook/whatever can trivially link all of your accounts through the SSO (and when you access your accounts). Of course, Google can get some of that

But Google/Facebook/whatever can trivially link all of your accounts through the SSO (and when you access your accounts). Of course, Google can get some of that information from GMail and both can get it from tracking (and GP was talking about the other way around), but it is something to consider.